Cumulative Record. Chapter 10: The Experimental Analysis of Behavior. Quote 19

In speaking about colors projected on the key or the fact that a key is on the right or left, we are, of course, talking about stimuli. Moreover, they are stimuli which act prior to the appearance of a response and thus occur in the temporal order characteristic of the reflex. But they are not eliciting stimuli; they merely modify the probability that a response will occur, and they do this over a very wide range. (p. 143)