Donders model is a biological explanation of behavior, the most complete definition of which is given in the English translation of the article by E.N. Sokolov “Animal Training”, first published in Russian in 1925: “A. Donders speaks of a state of excitement in the animal (disorder) caused artificially.” If conditioned salivary and skin reflexes in animals are “always, according to Donders’ definition, conditioned” and the animal’s behavior has a “conditional character,” then in animals with preserved unconditioned reflexes there is also an “unconditioned state of excitation of the nervous system.” So, “the more sharply the stimuli differ in size, the more consistently they excite this system; the less these centers are stimulated, the less excited the system is.” On paper it looks like this: the brain is like a detector of enemy faces. But before you test it yourself, it's important to understand how the detector works.