To clarify this metaphor: if a man has never had the experience of sight, there is no way to communicate the
experience to him. No discussion of light waves, retinas, rods, and cones could make sight meaningful to a man
who has been blind since birth. Like the basic attributes of physical objects, such as extension and mass, the basic
categories of consciousness can be defined only ostensively, i.e., by reference to direct experience. Just as
extrospective ostensive definitions are indispensable to any communication among men concerning the physical
world, so introspective ostensive definitions are indispensable to any communication concerning the psychological
realm. These extrospective and introspective observables are the base on which all more complex concepts, and all
subsequent, inferential knowledge, are built.
Introspection is the first source of one's psychological knowledge; and without introspection no other avenue of
psychological knowledge could be significant or meaningful, even if it were possible. The study of behavior, or of
the descriptive self-reports of other men, or of cultures and cultural products, would yield one nothing—if one had
no apprehension of such phenomena as ideas, beliefs, memories, emotions, desires, to which one could relate one's
observations and in terms of which one could interpret one's findings. (Strictly speaking, of course, it is absurd to
imagine that, if one had no awareness of such categories, one could be engaged in the study of anything.)
While introspection is a necessary condition and source of psychological knowledge, it is not sufficient by itself—
neither one's own introspection nor the introspective reports of others. Psychology requires the study of the outward
manifestations and expressions of mental activity: behavior. Consciousness is the regulator of action.
Consciousness cannot be fully understood without reference to behavior, and behavior cannot be understood
without reference to consciousness; man is neither a disembodied ghost nor an automaton. Scientific psychology
requires that the data of introspection and the observations of beings in action be systematically integrated into
coherent knowledge. A theory, to be valid, must integrate all and contradict none of the relevant evidence or data;
and this entails the necessity of taking cognizance of everything that is relevant.