The human brain is the greatest wonder Nature has ever wrought on this planet. Science was unable to cope with its astonishing complexity till the twentieth century. The great Russian physiologist Ivan Pavlov and his numerous disciples were responsible for the first major achievements in the study of the brain. Pavlov’s success may be explained by the fact that he fortunately chose to study a phenomenon which, on the one hand, could be regarded a simple physiological act and investigated by conventional physiological methods of research, but which was, on the other hand, a psychic phenomenon. Moreover, this phenomenon was later found to be the very elementary psychic act which, according to Pavlov, provides the cornerstone of the immense edifice of mental activity and became known as a conditioned reflex.
It would be an exaggeration to say that the theory of conditioned reflexes was generally recognized from the very start. The older generation of scientists have not forgotten the time when very few people believed that it would ever be possible to comprehend the extremely involved functions of the human brain. The situation has changed since then. But even now there are still those who doubt that mental activity is based only on systems of conditioned reflexes (or temporary connections), that is on extremely simple reactions in the organism.
Our brain doubtlessly has many mechanisms of mental activity so far obscure to us, but they are all dependent on conditioned reflexes. Any cell in the body, and, of course, any unicellular organism, is more or less capable of retaining traces of the previous stimulations and modifying its reactions according to past experiences, that is it is capable of associating one event with another. This function is more pronounced in the nerve cells whose development made it the prerogative of the nervous apparatus.