Ivan Pavlov, answered from the texts and cited to the page.
An animal lives among signals — the bell that means food, the footstep that means the keeper — and its whole higher nervous activity is the conditioning of such signals, the first signal system, which man shares with the beasts.1 But man has something the dog has not: the word. In man a second signal system is built upon the first — a system of words, which are signals of signals, standing not for this bell or that food but for whole classes of the first signals, and conditionable in their turn.
This is the physiological basis of everything that raises human thought above the animal's: of abstraction, of generalization in speech, of the capacity to be moved by what is said as by what is seen. The word is a conditioned stimulus of a new and higher order, and the laws of the conditioned reflex, which I established on the saliva of dogs, reach up through the second signal system into the highest reaches of the human mind — and into its disorders.
For if human higher nervous activity is conditioning in two systems, then the human neuroses and psychoses are disturbances of excitation and inhibition in those systems, of nervous type overstrained, and psychiatry becomes at last a branch of the physiology of the brain. The road that began with a dog's salivation runs, unbroken, to the clinic and to human speech.2
If you whistle, or ring a bell, or raise the hand, or scratch the dog — whatever^ you will — and now give the dog food and repeat this several times, then each one of these stimuli will evoke the same food reaction: the animal will strive toward the stimulus, lick his lips, secrete saliva, etc, — ^there will be the same reflex as before.Lectures on Conditioned Reflexes
Here is another example—the reflex of self-defence. The strong carnivorous animal preys on weaker animals, and these if they waited to defend themselves until the teeth of the foe were in their flesh would speedily be exterminated. The case takes on a different aspect when the defence reflex is called into play by the sights and sounds of the enemy's approach.Conditioned Reflexes
Pavlov's most ambitious extension — the claim that the conditioned- reflex framework reaches, through the "second signal system" of language, into the specifically human mind and its pathology. The doctrine became central to Soviet psychology and was engaged, and complicated, by Vygotsky and Luria's work on language and higher mental functions; the application to psychiatry descended into the behavior therapies. Against it stands the cognitive and linguistic objection, sharpest in Chomsky's critique of the conditioning account of language, that human syntax, productivity, and meaning cannot be built from conditioned word-signals — that language is not a second signal system in Pavlov's sense at all. The extension is thus both the boldest reach of the objective program toward the human mind and the place where it strains hardest against the distinctive structure of human thought.