Observational Terms Fred I. Dretske The Philosophical Review, Vol. 73, No. 1. (Jan., 1964), pp. 25-42. Stable URL: http://links.jstor.org/sici?sici=0031-8108%28196401%2973%3A1%3C25%3AOT%3E2.0.CO%3B2-S The Philosophical Review is currently published by Cornell University.
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OBSERVATIONAL TERMS
A
PIVOTAL distinction in contemporary accounts of science and the logic of scientific theories is that between two vocabularies or types of concept: the theoretical and the observational. Despite the importance of this distinction, and despite the fact that the status of theoretical terms depends, in many analyses, on their special relation to observational terms, one generally finds a very perfunctory discussion of what it means to call something an observable. The following is typical: "In regard to an observational term it is possible, under suitable circumstances, to decide by means of direct observation whether As it stands, the term does or does not apply to a given ~ituation."~ such a characterization is worthless. Given a liberal enough interpretation of "suitable circumstances" almost anything qualifies as an observable; that is, one might construe the suitability of the circumstances in terms of the possibility of deciding by "direct observation" whether the term applied or not. The fact that these circumstances could not be realized would not affect the "observability" of that to which the term applied. Moreover, since direct observation of something or other is presumably involved in deciding whether any term applies to a situation, some indication must be given of what it is that shall be directly observed. For example, in watching the gold leaves of an electroscope diverge, are we telling "by means of direct observation" whether the term "electric charge" is applicable to this situation? In one sense we certainly are (there are no mirrors, microscopes, and the like), and therefore electric charges should, according to Hempel's formula, be treated as observable. O r must we, rather, observe the referent ofthe term itself (in some appropriate sense of "referent") ? If so, what does it mean to observe something "dire~tly"?~ Carl Hempel, "The Theoretician's Dilemma," in Herbert Feigl, Michael Scriven, and Grover Maxwell (eds.), Minnesota Studies in the Philosophy of Science (Minneapolis, 1958), ii, 42. Also see pp. 164-165 in "Problems and Changes in the Empiricist Criterion of Meaning," reprinted in Leonard Linsky (ed.), Semantics and the Philosophy of Language (Urbana, 1952). Cf. Henry Margenau and R. B. Lindsay, Foundation of Physics (New York,
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Questions such as these are usually given wide berth by a question-begging definition or by an assumption that one has at one's disposal, or someone has at his disposal, a tidy list of observational terms. Reference to the "common-sense" level of discourse or a "thing language" supplemented by several unilluminating examples is supposed to make clear what is and is not But do we not commonly "feel" the force of gravity, ~bservable.~ cc see" that a person is angry, "see" that a piece of iron is magnetized, and "feel" a temperature change? Are we to countenance as observable such items as temperature gradients, gravitational forces, anger, and magnetization simply on the basis of these ordinary l o c ~ t i o n s ?If~ so, how shall we draw the line between such notions and those (for example, electrons, psi functions, light waves) which are, if we can judge by the recent literature, unobservable? Simply on the basis that we do not say we see 1g57), p. 401: "With a system are associated a great number of properties, or quantities, such as position, momentum, energy, and the like capable of measurement. These will be called observables, although it is not intended to imply that they are observable directly." In what sense, then, are they observable? Rudolf Carnap, "Logical Foundations of the Unity of Science," in Otto Neurath, Rudolf Carnap, and Herbert Morris (eds.), International Encyclopedia of Unijied Science (Chicago, 1g55), vol. i, pt. i, pp. 52-53. See also Carnap's "The Methodological Character of Theoretical Concepts," in Feigl et. al. (eds.), oP. cit., pp. 40-42. Carnap provides as examples of observational terms such predicates as: "hot," "cold," "heavy," "light," "red," "blue," cc small," and "thin." The question is: in regard to what are such predicates observational? That is, are the terms "heavy molecule" and "light molecule" to be taken as observational, or only "heavy chairs" and "light chairs"? Are white corpuscles and red corpuscles observable or are these predicates to be taken as observable only when applied to something more "immediate"? Other authors simply describe an observational term as one that refers to things which can be observed. Cf. Ernest Nagel, The Structure of Science (New York, 1961),p. 350. This, obviously, doesn't take us far. * Not according to Carnap; terms like "hot" and "cold" may be regarded as belonging to the thing language, but not "temperature" because "its determination requires the application of a technical instrument" ("Logical Foundations of the Unity of Science," pp. 52-53). What about temperature difference? Do we really need a technical instrument to tell that there is a temperature difference between cold water and hot water? Also, we are not told what constitutes a "technical instrument." Is "length" in the same class with temperature because its determination (i.e., what the length of anything is) requires the use of a technical instrument-a yardstick?
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(hear, feel) them? But why do we not say it? And what if some people do say it? Is this just a matter of linguistic custom? We could, of course, ignore ordinary language. To do so, however, is to run the risk of a gradual reductionistic erimination of everything we normally consider observable. That is, if we use, as our basis for rejecting magnetization as an observable, the fact (if it is a fact) that it is an inference, or can be so regarded, from the behavior of iron filings in the presence of the magnetized object, the same sort of objection can be made (and often is made) to treating iron filings and their behavior as themselves observable. The inference, in this case, relies on the assumed association of certain chemical and physical properties with the metallic-like flakes which are observed. According to this familiar account, one does not (strictly speaking) see iron filings arranged in a certain fashion; one sees a certain pattern of gray particles which are interpreted as iron filings. This sounds strange, I grant, but we should expect strange things when we ignore the familiar. In the following sections I want to examine the kinds of things we do say we see, not the kinds of things some philosophers tell us we ought, in all strictness, to say we see. Specifically, I want to delineate one particular sense of the verb "to see," a sense which does, I think, approximate the more technical locutions "to observe" or "to observe directly" as they are used to explicate the notion of an observable in science and the philosophy of science. I am interested not only in what we say we see, but also in why we feel it natural to say we see some things and why we find it odd or strange to say this about other things. Such an inquiry should, as I hope this one will, be preliminary to investigations into the logic of theoretical concepts.
I am interested in one special sense of the verb "to see." It is, if I am not mistaken, the sense of the term which philosophers have in mind when they talk about observational terms. Two features distinguish this use of the verb. First, it is a success word. That is, one cannot see X if X does not exist; if it is true that Y sees X, then there is an X which T sees. Similarly, if Y sees that
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S is P (or that S, is related by R to S,), then S is P (S, is related by R to S,). Macbeth did not see a dagger before him in this sense of the term. This is not to deny, however, that he said (or thought) he saw (in this sense of "to see") a dagger; one can claim success, in this sense, without having achieved it. Second, to say that one sees an X (or that S is P, or that S, stands in the relation R to S,) in this sense implies that one is looking at (in the direction of) an X (or S, or S, and S,). Subject to a qualification to be stated in a moment, this requirement prohibits us from saying we see (in this sense) those things at which we are not looking or those things at which it makes no sense to suppose we are looking. For example, one hears such statements as "I see that today is Tuesday," "I see that the stock market is falling," and "I see what you mean." In each of these cases the directional element is missing; one cannot look in the direction of today, the stock market, or someone's meaning. The sense of "to see" in such cases is different from the one in which I am interested. In the sense of the term which I am trying to characterize, one can say that one sees a tree (for we can look at-in the direction of-the tree), but one cannot see that there is a prime number between twenty-one and twenty-five. Of those things at which it does make sense to say we are looking, I have said that we must be looking at them to say truly we see them. This needs some explanation. The prevalence of various reflecting, refracting, and transmission devices (mirrors, periscopes, television, and so forth) have given currency to extended notions of "seeing" when we are not looking in the direction of the things we claim to see.5 Clearly, such uses are excluded from the more restricted sense of "to see" which I have just characterized. Nevertheless, the decision to include or exclude this extended notion of "to see" will not materially affect the type of thing that is observable; rather, it will alter the occasions on which that which is observable is actually observed. That this is so can be seen from the fact that such special devices as mirrors do not permit us to "see" that which, on other occasions and in I exclude, for the moment, such amplifying devices as telescopes, microscopes, etc.
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different circumstances, cannot be seen in the more restricted sense (recall footnote 5). I may see an automobile approaching through a rear-view mirror; I may see a battleship through a periscope or system of prisms; and I may see a play on television; but these are the p p e of things (automobiles, battleships, plays) which can already be classified as observable on the basis of our more restricted sense of "to see." Hence, I feel justified in operating with the more limited notion since it is my purpose to shed some light on the general concept of an observable, not on the question of on which specific occasions that which is observable is, in fact, ~ b s e r v e d . ~ Notice, by the way, that these two conditions (the existential and the directional) are necessary, not sufficient, for truly asserting that one sees (in this special sense) such and such. I n particular, there is no necessity (obviously) for that which exists to be seen; similarly (but not so obviously) there is nothing in what I have said that would require an observable to be seen when one is looking in its direction (or looking at it). I have, in other words, left it an open question whether, say, large molecules are observable; one can look in their direction without seeing them, although I am committed to saying that if one does see them (through some special device) one must be looking at them. That this sense of "to see" is a common one can be appreciated by noting some of the standard objections to a person's claim to see an X (or that p) : ( I ) You cannot be seeing an X; there aren't any (or there aren't any in this vicinity). ( 2 ) How can you see that S is P? You aren't even looking in the right direction; S is over there. The capacity of most of our perceptual reports to be challenged in this fashion indicates that both the existential and directional conditions are operative. Furthermore, it is this sense of "to see" which is of direct relevance to discussions of the observational terms" of science; that is, although these two 66
Henceforth, whenever I use the verb "to see," it will be in the sense just given unless stated otherwise or flagged by quotation marks. I realize that the concept of an "observable," as it is generally used in the literature, comprehends all the sense modalities. It should be clear by now that I am interested only in that sense of "observable" which pertains to the sense of sight. Additional complications, some of a fundamental sort, would arise if the discussion were to be made more general.
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conditions are not jointly sufficient for "observing" something (sufficient conditions will be discussed in the following section), part of what is meant in classifying a term X as an observational term is that in observing X we must be looking at X and X must exist. I cannot prove this; I should not know where to begin. Considerations such as the following, however-and they could easily be multiplied-do lend support to this view. A scientist, working in his laboratory, can see his test instruments; almost everyone would grant this much. O n the other hand, many would deny that he sees that which he measures with many of these instruments. There is a reluctance (to put it mildly) to saying that the electronics engineer sees the potential difference which he measures by the use of a voltmeter. We say he sees what the potential difference is, or that the potential difference is such and such, but not the potential difference itself. Part (but only part) of the reason for this is, I suggest, that the direction in which one looks to find out what the potential difference is (or that it is such and such) is completely arbitrary in relation to the place where the potential difference lies; one need only look at the meter. Whether one happens to be facing the direction in which the potential difference lies is of no consequence.' The ready classification of "voltmeter" as an observational term and "potential difference" (as determined by such a device) as a nonobservational term is, to some extent, understandable if we construe "observable" on the model of our restricted sense of "to see." More must be said, however, about our reluctance to classify such terms as "potential difference" as observational; their directionless character (as determined by meters) is only part of the answer. The following section contains a closer inquiry into the conditions governing this sense of "to see." If I am correct in supposing that it is this sense of the term which regulates the "observability" (in its philosophical-scientific use) of things, then an investigation into the remaining conditions constituting the correct and 7 This is not to deny the possibility that there may be other detection systems which would permit us to say we saw the potential difference-i.e., detection systems in which the directional element was present.
30
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appropriate use of this term should shed light on what is and is not observable.
Expressions of the following type are frequently used in contexts in which the sense of the verb "to see" is that sketched in the last section: ( I ) "I can see that Mr. X is taller than Mr. Y." ( 2 ) "I see that Otto has a headache." (3) "See how the light waves bend as they pass through this prism." There are, of course, situations in which such expressions would be inappropriate or -in the case of the statements ( I ) and (2)-false. To take the second example, it would be false to assert that one could see that Otto had a headache if one knew, for one reason or another, that he did not have one. I t would be false even though one saw him displaying some of the symptoms normally associated with having a headache. O n the other hand, it would be inappropriate to assert that one could see that Otto had a headache if one had been antecedently and pointedly informed of Otto's penchant for dissembling in just those circumstances in which one now finds him.8 Otto might have a headache, but it would be perverse in such a case to insist that one could see that he had one. Similarly, there are circumstances in which it would be inappropriate (or false) to say that one could see that Mr. X was taller than Mr. Y. I t would be false, for example, if Mr. X and Mr. Y were not roughly perpendicular to your line of sight and in fairly close proximity both to you and to each other (although these conditions would vary with the degree of difference in their height and other factors). It would be false because to say that one can see that Mr. X is taller than Mr. Y is, in part, to give the listener to understand-to imply-that one is in a position to make such Although inappropriate, it might not be false. One might know Otto, and the circumstances in which he is inclined to do certain things, better than one's informant. The inappropriateness of asserting that one can see that he has a headache stems from the fact that the satisfaction of the existential condition (that he does have a headache) has been indirectly challenged by the information received; to assert, in spite of this, that one can see that he has a headache is to ignore the suspicion cast upon one's ability to do just that. This point will be referred to again later.
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a judgment; that one is, to some extent, confident that the conditions are satisfied for reliabb detecting (discriminating, distinguishing) the dzxerence in question simp& b~ looking. If one knows that these conditions are not satisfied, then it is false to say that one sees that Mr. X is taller than Mr. Y, even if, in point of fact, Mr. X is taller than Mr. Y and one knows it. If one has reasonable grounds for doubting that these conditions are satisfied, then it becomes inappropriate (though not necessarily false) to assert that one can see their difference in height. These remarks can be amplified by means of a somewhat oversimplified model. A necessary part of any context in which the verb "to see" (in our special sense) is correctly (that is, both appropriately and truly) used is a certain body of beliefs on the part of the speaker; call them S-beliefs. These beliefs concern the conditions-call them S-conditions-which must be satisfied before one can reliably detect (distinguish, identify), merely by looking, that which one claims to see. To see an X is to have something look like an X in conditions which make the look of the situation a reliable index to the way things actually are. The belief that these conditions are satisfied is what makes it appropriate to confer upon oneself-verbally-the success implied by one's use of the verb "to see." Whether one's claim to success is justified or not depends not only on the belief that these conditions are satisfied, but also on the fact that they are satisfied. Challenges to one's claim to see X (or that p) are frequently met by falling back on a claim to see T (or that q) and, depending on the nature of the objection, using this as a basis for inferring X (or that p). Such challenges obtain their foothold in the field of S-conditions; they question or deny that certain S-conditions are, in fact, satisfied. In other words, they constitute a denial that, in these particular circumstances, there is any reliable correlation between the way things look to the speaker and the way things actually are. "You are too far away to tell," "You aren't even looking in the right direction," "But you don't have your glasses on," and "You can't possibly see it from this angle" all embody direct challenges to specific S-beliefs. Generally, the S-beliefs associated with seeing Y (or that q) are a proper subclass of those associated with seeing X (or that p). At the very least,
OBSERVATIONAL TERMS
they exclude those "questioned" beliefs implied in one's original claim to see X (or that p). Hence, the rejoinder to such challenges is generally immune from the initial objection since it eliminates (does not presuppose) those S-beliefs (the satisfaction of those S-conditions) which were the basis of the original challenge. Of course, doubting can continue-at least in principle. With each successive challenge the class of S-beliefs shrinks until, presumably, it is null. I t no longer makes sense to question the perceptual report since no conditions need be satisfied reliably to detect, merely by looking, that which one claims to be seeing. I n this limiting case, the claim to see X, lacking the S-conditions associated with common instances of seeing, is tantamount to a report on how things look to the speaker since there are no S-conditions to separate the "looking" from the "seeing." This, according to some philosophers, is the level of "direct" observation. I have already refused to enter this terrain. I am interested in those senses of "to see" in which we see people, tables, and oscilloscopes. Such reports imply a belief context such as that just described. One can certainly doubt that one sees a chair, and sometimes this doubt is reasonable, but this does not mean that one never sees a chair. A reasonable confidence in the satisfaction of the S-conditions, coupled with what looks to be a chair, is an appropriate setting for saying that one sees a chair. If these S-beliefs are challenged, it may become inappropriate to continue in the asseveration that one sees a chair, but there is no reason to deny that one sees a chair. The latter move can be supported only by showing that, as a matter of fact, one or more of the S-conditions are not satisfied. Doubt alone will not achieve this. When questioning someone's claim to see X (or that p), one is denying that the person can reliably detect or identify, simply by looking, that which he claims to see; one's reason for issuing the challenge resides in one's belief that some of the conditions are not satisfied for his (not necessarily anyone else's) doing what he claims to be doing. Nevertheless, because we deny a perceptual report on the basis of our conviction that one or more of the S-conditions are not satisfied, this should not be taken to mean that the perceptual report embodied an assertion that these Sconditions were satisfied. Rather, it implied that they were satisfied
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in the same sense in which someone's claim to have jumped over a fence would imfiZy that he did not have two broken legs. We might, to carry the analogy a bit further, deny that someone jumped over a fence on the basis of information regarding his broken legs, but no one, I should think, would maintain that his claim to have jumped over the fence was, in addition, an assertion about the fitness of his legs. In both cases the report of an achievement is denied, not because it asserted that conditions were satisfied which were not in fact satisfied, but because there is an assertion that something was done which the challenger has reason to suppose, on the basis of information regarding the limitations of certain people in certain circumstances, could not have been done. The regular satisfaction of the S-conditions must, in the setting in which the perceptual claim is made, be relatively uncontroversial in nature. Not only must one believe that the S-conditions are satisfied, but their satisfaction on occasions of this general type should be a fairly routine matter and one about which dispute is neither active nor expected. The satisfaction of the S-conditions for seeing X must be common enough, in other words, to permit the perceptual report itself to occupy the focus of attention and not the conditions which it implies are satisfied. Unless this were the case there would be no point in saying one could see X. For one is, as it were, supplementing certain presumptions concerning the satisfaction of relevant conditions with a report on how things look in those conditions. One is not, of course, reporting on how things look since this sort of language is fitting only when the satisfaction of the S-conditions is questionable. To describe how things look to you and to describe what you see are two different things; but the difference lies not in the look of the situation, so to speak, but in the S-beliefs one brings to that situation. "See" language signals the fact that certain assumptions are being made which the speaker feels it reasonable and natural to make in that context; unless such assumptions were routine and uncontroversial, a perceptual report could never be built upon them. There are two remaining points which I should like to make before discussing the implications of this analysis for the question
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of "observables." A distinction which I have so far indicated but not explicitly discussed is the difference between seeing S and seeing that S is P (or that S, is R to S,). Although it might be eminently reasonable to say that one sees that S is P or that S has P, it might be odd to say that one sees P's or S's P. For example, one sees that a person has a headache, but one does not see his headache. We find it odd to say that we see a person's headache because, I think, we refuse to particularize the headache; it is a state or condition which the person is in, not something he carries about with him which we could, as it were, see independently of him. Whatever the basis for this distinction, it is important and should be kept in mind. As an illustration of how it might cause confusion, consider the notion of "mass." One is inclined to say that we do not see or feel mass. Yet we do feel a dzxerence in mass and (as I shall argue more fully later) even see a difference in mass when the two masses are properly arranged on a balance. "Mass" is a relational concept as are most of the concepts of science; hence, if anything connected with the concept "mass" is to be construed as an observable, it will be mass difference; that is, one will see that one mass is greater than another, not mass itself, whatever that might mean. Finally, one is too often, I suspect, prepared to classify an X as unobservable purely on the grounds that nothing with which one is acquainted looks like an X. One does this before one has given careful consideration to what X's are supposed to look like. When one sees that Otto has a headache, what does he look like? What is the look of the situation which allows us to say this? Answer these questions-and the answers will vary with the circumstances in which Otto is placed-and you have answered the question: what does Otto's having a headache look like? To anticipate myself slightly, one might ask oneself this question about light waves (electromagnetic waves of a certain wave length). If there are no conditions under which we say we see light waves, then we cannot answer this question, as we did above, by noting the appearance of the situations in which we say we see them. But it would be a confusion to say that we cannot see them because we never, in all our experience, see anything which looks like light waves. This, it seems to me, is putting the
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cart before the horse. If we ever found it appropriate to say that we see light waves-and I shall argue in the next section that there are contexts in which it would be appropriate-then that would determine what light waves look like; they would look as things looked (in the direction we happened to be looking) when we said we saw them. I shall come back to the matter of light waves a little later; there is nothing, as far as I can tell, in what I have so far said which would preclude our euer seeing light waves.
With this much groundwork we are in a position to explore the implications concerning observables. Clearly the distinction between observables and unobservables is not drawn, as some would have us draw it, in terms of the presence or absence of S-conditions (sometimes mistakenly referred to as interpretive or inferential elements in observation) since these are present in almost every case. I t would seem, then, that the distinction must lie in the nature of these conditions themselves. That is, we look to see if there is a red chair in the room, or we look to see if Mr. X is taller than Mr. Y. If we are assured that certain conditions are satisfied (the fact that we look indicates that we think they are satisfied), we say we see the red chair in the room or see that Mr. X is taller than Mr. Y. I n other cases people look to see which of two masses is greater and look to see if there is an electric charge on the electroscope; but we are disinclined, or some people are disinclined, to say that we see the relative mass or the electric charge-even if the conditions are satisfied (and we believe them satisfied) for making a reliable identification or detection just by looking. Reluctance in this matter must stem from the type of conditions that must be satisfied. In both cases some conditions must obtain, and in both cases we decide the matter by looking. Why, then, do we not say we see, in every such instance, that which we detect by l ~ o k i n g ? ~ Our previous discussion provides an answer to this question: the sort of thing one can see is relative-relative to that body I am aware that some people do say they see these things; part of what I am trying to do is account for this disparity in usage.
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of accepted scientific truth which reaches a degree of familiarity sufficient to allow one to construct a perceptual report on the presumption of its truth. The acquisition and dissemination of scientific knowledge provides the scientist, and to a far smaller degree the layman, with a framework of beliefs which appreciably alters that which he detects (identifies) simply by looking-that which he sees. I t is not, however, the mere belief in these regularities which is important; it is the extent of their familiarity and mechanical-like use in assessing and appreciating the significance of various phenomena which leads, eventually, to their incorporation into the background, as it were, of perceptual activity. When such beliefs attain this level of presumption, they restructure the look of the situation in a way which allows the person for whom they have attained this status to see that which he detects (on the presumption of these regularities) simply by looking. There is no absolute and static limit to the kinds of things one can observe, not unless there is an absolute limit to what can reasonably and routinely be presumed to be true in various contexts-that is, to one's S-beliefs. To use our previous example, does one see the difference in mass between two ordinary sized objects? When the objects are placed in opposite pans of a calibrated pan balance, one does detect their difference in mass simply by 1ooking.Anyone claiming to see the difference would, of course, be implying that he thought certain conditions were satisfied.1° He would be assuming, for example, that "g" was constant over the area of the balance, that the arms of the balance were of equal length, and so on. If such conditions were not satisfied, and the speaker did not believe them satisfied or had reason to doubt that they were satisfied, he could not anticipate the reliable correlation, requisite for his employment of the success word "to see," between the look of the situation (the balance) and the actual difference I am assuming here that anyone using the concept "mass" would be familiar with the relevance of certain conditions for its determination. Of course, one might say that one could see the difference in mass and be totally ignorant of these conditions (or some of them). I n a very real sense, such a person does not know what he is saying, or what it is that he is saying he sees; for he does not know what it is that such a statement implies.
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in mass. But there are situations ("suitable circumstances," to borrow Hempel's phrase) in which such assumptions are natural. Indeed, the whole point to calibrating and checking such an instrument is to enable one to see (reliably detect by looking) the differences it is designed to reveal. Furthermore, since in this particular case one must also be looking at the masses, there seems to be nothing preventing one from saying that the difference in question is seen. The temptation for some is to say that what we actually see is a difference in weight, not mass, or that, strictly speaking, one sees only two objects resting in opposite pans of an offset balance. Then, more or less automatically, one infers a differences in mass in accordance with certain scientific laws and theories. In some settings, of course, this is a perfectly appropriate way to describe what one sees: settings in which the relationships codified in the laws and theories are being tested, settings in which the relationships are suspected of not holding for some special reason, and so on. But that there are some situations in which this description is proper, does not mean that it is more proper in all situations. Consider light waves. We do not ordinarily say we see them by (although I think there are exceptions to this-notably physicists), but why not? When looking at a red object, for instance, why should we not say we see electromagnetic waves of such and such wave length (or see that the waves have such and such wave length)? The reason, I submit, is that the wave theory of light comprehends a set of relationships which are not familiar enough to the average individual, and most scientists, to permit their acceptance as S-conditions: that is, as relationships the regular satisfaction of which is a matter of course and taken for granted in the detection and identification of associated magnitudes. A person can detect, simply by looking, that there are light waves between him and the red object with a wave length of approximately 7,000 angstrom units, but the reason he cannot see them (alternatively, the reason he does not say he sees them) is that he is not familiar enough with this theory-if I may refer to all the relevant regularities as one theory-to permit the construction of a perceptual report on its routine assumption. Such routine assumption, as I am now using the phrase, com-
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prehends several things: ( I ) One must be thoroughly familiar with the theory and take its truth for granted. (2) One must know how, under a variety of conditions, light waves and associated magnitudes can reliably be identified and detected simply by looking (that is, one must know how to use this theory). And (3) one must be operating in an area where an acquaintance with, and utilization of, the distinctions and refinements embodied within this theory are essential to effective and efficient activity (that is, one must have a need to use the theory). Most of us have never reached this degree of familiarity; even if we should, it would still be the case that the majority of us have no need to utilize the refinements this theory affords in our day-to-day activities. Let me cite one of the many examples which could be used to illustrate this analysis. Meteorologists see cold fronts approaching. I do not. My eyesight is above average, but my familiarization with the relevant regularities is below the level requisite to see what they do. I am ignorant of certain things which the meteorologist not only believes but takes for granted when he assesses the look of the situation and reports what he sees. I see cloud formations of a certain type; so does he, but he sees more. To deny that he does is to question or challenge the satisfaction of certain conditions (which on some occasions may be quite justifiable)conditions the routine satisfaction of which permits him reliably to identify, simply by looking in the appropriate direction, that which he claims to see. More generally, what I am suggesting is that the principal factors preventing most of us (but not always all of us) from seeing those things which many feel we cannot and will never see are: ( I ) the lack of familiarization with the appropriate body of scientific information defining the conditions under which such things can reliably be detected simply by looking, and (2) the absence of a continuing need to utilize reference to such things in the progress of our day-to-day activities. I am not, to use our last example, trying to equate cold fronts with a certain type of cloud formation (definitionally or otherwise). That would be as much a confusion as the equation of a man's anger with his wild gesticulations. Nor am I saying that every-
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thing that is true of the one is true of the other-any more than I should maintain that anything that is true of a man's anger is true of his corresponding gestures. But I am suggesting that the difference between seeing such and such cloud formations and seeing the approach of a cold front is similar to the difference between seeing a man gesticulating angrily and seeing that he is angry. I n each case, the latter perceptual report implies the satisfaction of a different, and generally more complicated, set of S-conditions. This fact accounts for the report's greater exposure to challenge and, in some respects, for its greater anticipatory value. If I see, not simply that a man is gesticulating angrily, but that he is angry, I can be more accurate in certain projections of his subsequent conduct in modified or altered circumstances. By the same token, if I see, not simply a cloud formation of such and such type, but an approaching cold front, I have seen something whose various effects and subsequent appearance can be more accurately anticipated.
There are certain features in the origin and development of scientific theories which can easily obscure, or totally conceal, the validity of this type of analysis. Let me mention just one. There is often a model or analogy by reference to which a scientific theory is conceived and developed. This model generally consists of a familiar set of regularities obtaining between observable objects. Theoretical (unobservable) entities are then introduced on the analogy of these familiar objects and their observable behavior. Now, it is easy to suppose that if we could observe those entities about which the theory speaks, we should observe something closely resembling those familiar entities in the model. I t is natural to suppose, for example, that if we could see electromagnetic waves, we should see some vibrations of a peculiarly diaphanous sort. Or, depending on one's background, one might expect to "see" two perpendicular vectors advancing toward one as they oscillated from zero to peak amplitude. Their failure to "see" anything remotely resembling these processes
OBSERVATIONAL TERMS
leaves some with the impression that electromagnetic waves, describable for certain limited purposes in these analogical terms, are not visible. No matter how much one learns about the wave theory of light, one will not learn to see, when looking at light rays, the type of phenomenon just described. I should insist, however, that this is not how electromagnetic waves (within a certain span of wave lengths) look. I n certain situations they look, or would look if we saw them, as colored objects look. Clearly, if electromagnetic waves looked the same as did the processes which are used as their model, there would be no need for the model in the first place. Not everyone can see that we are depressed; only close friends, people with whom we have had a particularly intimate and prolonged acquaintance, can detect our moods though we try to conceal them. Not everyone can see a cold front approaching; meteorologists can. Nor can everyone see the various stages of many diseases; specialists can, and this is not simply a matter of knowing the name of something of which we do not know the name. I t also includes a knowledge of the way these things appear under a variety of conditions, a familiarity with the circumstances in which these conditions regularly obtain, and a need, for one reason or another, to utilize reference to such phenomena in daily intercourse. I t is an acknowledged fact that physicists, for example, sa_y they see things that the layman, including philosophers, do not: What physicists often call "observable magnitudes," e.g., mass, position, velocity, energy, frequency of waves, and the like, are not "observable') in the sense customary in philosophical discussion^.^^ What is not generally acknowledged, especially by philosophers, is that they often do see what they say they see, and this fact cannot be accounted for by disparaging their "loose" talk in these matters. They do see things which others, less familiar with their subject matter, do not. Why shouldn't they? There is no static and preordained upper limit to what people can see, and, mutatis Carnap, "The Methodological Character of Theoretical Concepts," P. 49.
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mutandis, there is no static limit to the observational vocabulary of science. This vocabulary is subject to the same development and expansion as is scientific knowledge itself.12
FREDI. DRETSKE University of Wisconsin
l2 I wish to thank my colleague, Gerald MacCallum, for reading and carefully criticizing an early draft of this paper.
