All by themselves at first indifferent, or as they are otherwise called, indefferentnye, irritants, coinciding in time with the action of the unconditioned, become conditioned to the animal, or signal, stimuli. They animal signals about upcoming events. "For the animal, – wrote Pavlov – the reality is signaled almost exclusively stimuli and their traces in the cerebral hemispheres, coming directly special cells in the visual, auditory and other receptors of the body. This is what we have in ourselves as impressions, feelings and views of the surrounding environment, as obscheprirodnoy and our social, excluding word, an audible and visible. This – the first signal system of reality, the total we have with animals. But the word was a second, specially our alarm system in fact, as a signal of the first signals.
" So way, a person has two alarm systems: one, shared with animals, and the second, especially human, which animals do not. What is the difference in the signaling of reality by means of signals of first and second signaling systems? Consider a few examples. During the , many have heard the sounds of engines of enemy bombers. Combined with the subsequent explosion of bombs, they became the signals bombing, the signals of danger. The same signal was also a siren. This alarm signal by the first signal system. But the siren could be replaced by a message on the radio: "air alert" – this alarm when signals using the second system. You are doing in my room.