Ivan Pavlov, answered from the texts and cited to the page.
The conditioned reflex — let me be exact about what it is, because loose talk has surrounded it from the beginning. Take any stimulus that is, in itself, indifferent to the animal: a whistle, a bell, the raising of a hand. Pair it repeatedly with food, and after several such coincidences the whistle alone will produce the full alimentary reaction — salivation, striving toward the source, the whole complex that food itself evokes.1
That is the conditioned reflex in its simplest form. But notice what the pairing requires: the signal must precede the unconditioned stimulus, not follow it. If the order is reversed — the food given first, the signal after — the connection either fails to form or becomes inhibitory.2 The temporal priority of the signal is not a detail; it is the law.
Why does the organism build such connections at all? Because the world is not a laboratory in which food arrives without warning. A sound from a distance that signals danger gives the animal time to save itself; a signal for food allows preparation before the food arrives.3 The conditioned reflex is, in short, the mechanism of the most delicate equilibration of the organism with its surrounding world — the higher adaptability, built during the individual's life upon the foundation of the inborn, unconditioned reflexes with which the animal is born.
And the system is not only positive. Alongside the positive conditioned reflex — which calls forth excitation in the cortex — stands the negative, which calls forth inhibition.4 When a conditioned stimulus repeatedly fails to coincide with the unconditioned reflex that formed it, it does not simply cease to work; it is actively suppressed by an internal inhibitory process.5
This internal inhibition is itself governed by exact laws: an indifferent agent that coincides repeatedly with the conditioned stimulus when the unconditioned reflex is absent will itself become a conditioned inhibitor, capable of suppressing other conditioned reflexes built on the same unconditioned base.6 Excitation and inhibition, their irradiation and concentration across the cortex, their mutual induction — these are the foundation of everything the hemispheres do.7
The whole of what the old psychology called mind — association, memory, attention, the recognition of signals — is this: the formation, stabilization, and extinction of temporary nervous connections, governed by physiological laws as definite as those that regulate any other phenomenon in the nervous system.8
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.Lectures on Conditioned Reflexes
A full and stable conditioned reflex develops when the stimulus which is to become conditioned slightly precedes that activity (unconditioned reflex) with which it is to be linked... if the stimulus is introduced after the beginning of the activity, then... the stimulus... becomes inhibitory.Lectures on Conditioned Reflexes
If some danger, for example, is signalled by a sound from a distance, then the animal will have time to save himself... The higher adaptability of animals, the most delicate equilibrations with the surrounding medium, are unfailingly connected with this kind of temporarily formed reflex.Lectures on Conditioned Reflexes
Alongside of the positive conditioned reflex we have the negative, which calls out in the central nervous system not a process of excitation but one of inhibition.Lectures on Conditioned Reflexes
If a certain agent, our conditioned stimulus, replacing and signalling food and provoking the corresponding reaction of the organism... proves to be in contradiction to reality, i.e., if it fails to coincide several times with eating, it gradually loses its stimulating effect. This result is brought about not through the destruction of the salivary reflex, but through its temporary inhibition by means of a special internal process.Lectures on Conditioned Reflexes
This additional, formerly indifferent agent, owing to which the conditioned stimulus in the combination has gradually lost its stimulating effect, we call a conditioned inhibitory stimulus... when joined to every other conditioned stimulus which is based on one and the same unconditioned stimulus, it inhibits it from the first test.Lectures on Conditioned Reflexes
As the foundation of the activities of the hemispheres we recognise the processes of excitation and inhibition, their movement in the form of irradiation and concentration, and their mutual induction.Lectures on Conditioned Reflexes
I believe that the recognition and the study of the conditioned reflex will throw open the door to a true physiological investigation probably of all the highest nervous activities of the cerebral hemispheres.Conditioned Reflexes