In 1903, at the 14th International Medical Congress in Madrid, Pavlov read a paper on «The Experimental Psychology and Psychopathology of Animals». In this paper the definition of conditioned and other reflexes was given and it was shown that a conditioned reflex should be regarded as an elementary psychological phenomenon, which at the same time is a physiological one. It followed from this that the conditioned reflex was a clue to the mechanism of the most highly developed forms of reaction in animals and humans to their environment and it made an objective study of their psychic activity possible.

Subsequently, in a systematic programme of research, Pavlov transformed Sechenov's theoretical attempt to discover the reflex mechanisms of psychic activity into an experimentally proven theory of conditioned reflexes.

As guiding principles of materialistic teaching on the laws governing the activity of living organisms, Pavlov deduced three principles for the theory of reflexes: the principle of determinism, the principle of analysis and synthesis, and the principle of structure.

The development of these principles by Pavlov and his school helped greatly towards the building-up of a scientific theory of medicine and towards the discovery of laws governing the functioning of the organism as a whole.

Experiments carried out by Pavlov and his pupils showed that conditioned reflexes originate in the cerebral cortex, which acts as the «prime distributor and organizer of all activity of the organism» and which is responsible for the very delicate equilibrium of an animal with its environment. In 1905 it was established that any external agent could, by coinciding in time with an ordinary reflex, become the conditioned signal for the formation of a new conditioned reflex. In connection with the discovery of this general postulate Pavlov proceeded to investigate «artificial conditioned reflexes». Research in Pavlov's laboratories over a number of years revealed for the first time the basic laws governing the functioning of the cortex of the great hemispheres. Many physiologists were drawn to the problem of developing Pavlov's basic laws governing the activity of the cerebrum. As a result of all this research there emerged an integrated Pavlovian theory on higher nervous activity.

Even in the early stages of his research Pavlov received world acclaim and recognition. In 1901 he was elected a corresponding member of the Russian Academy of Sciences, in 1904 he was awarded a Nobel Prize, and in 1907 he was elected Academician of the Russian Academy of Sciences; in 1912 he was given an honorary doctorate at Cambridge University and in the following years honorary membership of various scientific societies abroad. Finally, upon the recommendation of the Medical Academy of Paris, he was awarded the Order of the Legion of Honour (1915).

After the October Revolution, a special government decree, signed by Lenin on January 24, 1921, noted «the outstanding scientific services of Academician I.P.Pavlov, which are of enormous significance to the working class of the whole world».

The Communist Party and the Soviet Government saw to it that Pavlov and his collaborators were given unlimited scope for scientific research. The Soviet Union became a prominent centre for the study of physiology, and the fact that the 15th International Physiological Congress of August 9-17, 1935, was held in Leningrad and Moscow clearly shows that it was acknowledged as such.

Pavlov directed all his indefatigable energy towards scientific reforms. He devoted much effort to transforming the physiological institutions headed by him into world centres of scientific knowledge, and it is generally acknowledged that he succeeded in this endeavour.

Pavlov nurtured a great school of physiologists, which produced many distinguished pupils. He left the richest scientific legacy - a brilliant group of pupils, who would continue developing the ideas of their master, and a host of followers all over the world.

In 1881, Pavlov married Seraphima (Sara) Vasilievna Karchevskaya, a teacher, the daughter of a doctor in the Black Sea fleet. She first had a miscarriage, said to be due to her having to run after her very fast-walking husband. Subsequently they had a son, Wirchik, who died very suddenly as a child; three sons, Vladimir, Victor and Vsevolod, one of whom was a well-known physicist and professor of physics at Leningrad in 1925, and a daughter, Vera.

Dr. Pavlov died in Leningrad on February 27, 1936.