Simple orienting mechanisms permit us to capture in one or two well-defined processes the entire behavioral repertoire of the animal. The familiar psychological categories of motivation, cognition, sensation, and perception are all fused in harmonious combination in a handful of elementary mechanisms that initiate action, respond to stimulation, and guide the organism toward things that will aid its survival and reproduction. Behavior such as the avoiding reaction of Paramecium – reversal of movement in response to a sudden “change for the worse” – makes sense as part of a system of reactions that combine to produce adaptive behavior. With animals like birds or mammals, on the other hand, it is more difficult to see how things fit together. Seeking simplicity, many researchers have sought for some kind of behavioral “atom” – like the “reflex” or the “operant” – as the key to behavior in general. While this tactic has produced much interesting new knowledge, it has not paid off quite as hoped. The real key is no behavior in particular, but rather how all fit together to yield adaptation to a niche.

Paramecium can vary only its rate and direction of movement. Its rather rich sensitivity to a variety of chemical, photic, and thermal stimuli must, therefore, be funneled into this limited repertoire. “Higher” animals are not so restricted. Not only can they sense more aspects of the physical environment, they can do many more things in response. The avoiding reaction of Paramecium is one extreme of a continuum of reactions that guide an animal to a congenial environment. The reflexes of higher animals serve a similar function. Most reflexes avoid, escape from, or minimize the effect of noxious stimuli. But rather than being modulated by the environment, like the avoiding reaction, they are simply replaced by quite different kinds of behavior when circumstances demand it. The boxer reflexly blinks and ducks as his opponent's fist approaches his face, but if he avoids successfully, his next reaction is likely to be a planned offensive strategy, not another reflex.

Thus, reflexes are a part, actually a rather small part, of the adaptive repertoire of higher organisms. Nevertheless, historically they have had a special role because of the apparent simplicity of the relation between stimulus and response. The immediacy of reflexes, and their relative independence of the animal's past history, makes them easy to study.