{1} Ayer, A.J. Foundations of Empirical Knowledge. London: Macmillan, 1940, and "The Terminology of Sense Data" in Philosophical Essays, pp. 66-104. London: Macmillan, 1954. Also in Mind, 54, 1945, pp. 289-312.

{2} A useful discussion of views of this type is to be found in Roderick Chisholm's "The Theory of Appearing," in Max Black (ed.), Philosophical Analysis, pp. 102-18. Ithaca: Cornell Univ. Press, 1950 and in H.H. Price's Perception. London: Methuen, 1932.

{3} Broad, C.D., Scientific Thought, London: Kegan Paul, 1923.

{4} (Added 1963) When John has mastered looks talk he will be able to say not only "The tie looks green" but "The tie looks to be blue," where the latter has the sense of ". . . looks as blue ties look in these circumstances." The distinction between "looks ø" and "looks to be ø" corresponds to Chisholm's distinction between non-comparative and comparative "appears" -- statements.

{5} (Added 1963) Standard circumstances are, indeed, the circumstances in which things look as they are. But the non-trivial character of the above formula emerges when we replace "standard circumstances" by the mention of a specific kind of circumstance (e.g. daylight) and add that daylight is the standard circumstance of perception, i.e. the condition in which color words have their primary perceptual use.

{6} (Added 1963) The argument can admit a distinction in principle between a rudimentary concept of "green" which could be learned without learning the logical space of looks talk, and a richer concept of "green" in which "is green" can be challenged by "merely looks green." The essential point is that even to have the more rudimentary concept presupposes having a battery of other concepts.

{7} (Added 1963). . . and if S knew that the circumstances were normal.

{8} (Added 1963). . . and if the subject knew that the circumstances were normal.

{9} (Added 1963) The term "experiencing" in the question "Is the common descriptive component an experiencing?" is used in an epistemic sense. In the non-epistemic sense of an "undergoing," the common descriptive component is, of course, an experiencing.

{10} For a systematic elaboration and defense of the following interpretation of Locke, Berkeley, and Hume, the reader should consult "Berkeley's Critique of Abstract Ideas," a Ph.D. thesis by John Linnell, submitted to the Graduate Faculty of the University of Minnesota, June, 1954.

{11} I say that Locke was "on the whole" committed to the view that there can be an idea which is of the genus without being of any of its species, because while he saw that it couldn't be any one of the species to the exclusion of the others, and saw no way of avoiding this except by making it of none of the species, he was greatly puzzled by this, for he saw that in some sense the idea of the genus must be of all the species. We have already noted that if he had admitted disjunction as a principle of compounding ideas, he could have said that the idea of the genus is the idea of the disjunction of all its species, that the idea of being triangular is the idea of being scalene or isosceles. As it was, he thought that to be of all the species it would have to be the idea of being scalene and isosceles, which is, of course, the idea of an impossibility.

It is interesting to note that if Berkeley had faced up to the implications of the criterion we shall find him to have adopted, this disjunctive conception of the generic idea is the one he would have been led to adopt. For since being G -- where 'G' stands for a generic character -- entails being S₁ or S₂ or S₃ . . . . . or S_n, -- where 'S₁' stands for a specific character falling under G -- Berkeley should have taken as the unit of ideas concerning triangles, the idea of the genus Triangle as differentiated into the set of specific forms of triangularity. But, needless to say, if Berkeley had taken this step, he could not have thought of a sensation of crimson as a determinate thought.

{12} For an analysis of the problem of abstract entities built on this interpretation of semantical statements, see my "Empiricism and Abstract Entities" in Paul A. Schlipp (ed.), The Philosophy of Rudolph Carnap. Wilmette (Ill.), 1963; also "Abstract Entities," The Review of Metaphysics, June, 1963.

{13} (Added 1963) My thought was that one can have direct (non-inferential) knowledge of a past fact which one did not or even (as in the case envisaged) could not conceptualize at the time it was present.

{14} Ryle, Gilbert, The Concept of Mind. London: Hutchinson's University Library, 1949.

{15} Carnap, Rudolph, Introduction to Semantics. Chicago: University of Chicago Press, 1942.

{16} See "Truth and Correspondence," Part I; also "Empiricism and Abstract Entities" in Paul A. Schilpp (ed.) The Philosophy of Rudolph Carnap. Library of Living Philosophers Evanston (Ill.), 1963. (Available in mimeograph from the author.)

{17} An earlier attempt along these lines is to be found in "Mind, Meaning and Behavior" in Philosophical Studies, 3, pp. 83-94 (1952), and "A Semantical Solution to the Mind-Body Problem" in Methodos, 5, pp. 45-84 (1953).

{18} Campbell, Norman, Physics: The Element. Cambridge: Cambridge University Press, 1920.

{19} Carnap, Rudolph, "Thc Interpretation of Physics," in H. Feigl and M. Brodbeck (eds.), Readings in the Philosophy of Science, pp. 309-18. New York: Appleton-Century-Crofts, 1953. This seiection consists of pp. 59-69 of his Foundationss of Logic and Mathematics. Chicago: University of Chicago Press, 1939.

{20} Reichenbach, H., Philosophie der Raum-Zeit-Lehre. Berlin: de Gruyter, 1928, and Experience and Prediction. Chicago: University of Chicago Press, 1939.

{21} Hempel, C.G., FundamentaIs of Concept Formation in Empirical Science. Chicago: University of Chicago Press, 1952.

{22} Braithwaite, R.B., Scientific Explanation. Cambridge: Cambridge University Press, 1920.

{23} Skinner, B.F., "The Operational Analysis of Psychological Terms," Psychological Review, 52 (1945):270-77. Reprinted in H. Feig] and M. Brodbeck (eds.), Readings in the Philosophy of Science. New York: Appleton-Century-Crofts, 1953: pp. 585-94.

{24} Carnap, Rudolph, "Psychologie in Physicalischer Sprache," Erkenntnis, 3 (1933: 107-42.

{25} Wittgenstein, Ludwig, Philosophical Investigations. London: Macmillan, 1953.

{26} For a discussion of some logical points pertaining to this framework, the reader should consult the essay, "The Concept of Emergence," by Paul E. Meehl and Wilfrid Sellars, in Volume I of the Minnesota Studies in the Philosophy of Science, edited by Herbert Feigl and Michael Scriven. Minneapolis: University of Minnesota Press, 1956: pp. 239-52.