1. ALLPORT, F. H. Theories of perception and the concept of structure. New York: Wiley, 1955.
2. BERGMANN, G. and SPENCE, K. W. Psychophysical measurement. In M. Marx (Ed.) Psychological theory. New York: Macmillan, 1951. Pp. 256–275.
3. CARNAP. R. Philosophy and logical syntax. London: Kegan Paul, 1935.
4. CASSIRIR, E. An essay on man. New Haven: Yale Univer. Press, 1944.
5. CASSIRIR, E. The problem of knowledge. New Haven: Yale Univer. Press, 1950.