Even if we manage to become reconciled to this situation, however, our troubles are not yet at an end. There still remains the difficulty in certain cases of showing that the event which is selected as stimulus or cause bears any significant relationship to the event which figures in our scheme as the response. The stimulus is supposed to have a causal connection with the response, but how are we to know that this is the fact? How are we to know that the engineer who solves a problem for me at my request might not have done so anyway? No behaviorist can possibly show that the air waves set in motion by my vocalization were an indispensable stimulus. We doubtless believe that the spoken word was in fact the spark which lit the fuse and finally exploded the mine, but this belief involves a complication of causes which it is wholly beyond our power to control or to verify.
It is true, of course, that we are able, as a matter of fact, to correlate stimulus and response. I know that it was the spoken word which caused the commission to be executed, for the expert reminds me of the fact and presents a bill. But neither of us makes any pretense that his belief is derived from a scrutiny of the causal sequence. Memory furnishes us with a shortcut to the result. While our present acts are doubtless connected with the past through causation, we do not regard them as simply the effects of antecedent causes. They are rather responses to present stimuli. The expert presents his bill, being moved thereto by a stimulus which may be indicated by saying that it is the spoken-word-constituting-a-commission-now-completed-and-entitling-me- to-compensation. That is, the stimulus cannot be pushed back and anchored at a fixed point in the past, but is a present factor at the moment of response and is operative by virtue of its anticipation of future events.
If, then, psychology is to be regarded as a study of behavior, it is plainly necessary to reinterpret the category of behavior. For example, a purely mechanical response to a light-stimulus may properly be viewed as response to the ether-vibration or wave-length upon which it follows in temporal sequence. But if this stimulation results in what is commonly called consciousness, a different kind of response ensues. The light-stimulus becomes a cause or occasion for the act of looking. But why look, unless it be to secure a new stimulus for further response? We stop to look, precisely because the first stimulus does not run smoothly off the reel. The response will not go forward, but is halted and expends itself in the effort to secure a further stimulus. This is the moment of attention, in which the stimulus undergoes a process of transformation, concomitantly with the process of reorganization in the motor responses, and in the direction of ends or results that are foreshadowed in it. This change in the stimulus takes place under certain specifiable conditions, and the study of these conditions is a study of such processes as perceiving, attending, remembering, and deliberating, which are distinctively psychological in their nature. Processes of this kind, if taken as changes in stimuli, find an objective criterion in the adaptive behavior for the sake of which they occur, and they provide psychology with a distinctive task and subject-matter.