It is customary to divide human movements into three classes, voluntary, reflex and mechanical.
The physiologist can discover that both depend upon the nervous system, and he may find that the movements which we call voluntary depend upon higher centres in the brain than those that are reflex.
There is need for some care if the distinction between mechanical and vital movements is to be made precise.
Physicists distinguish between macroscopic and microscopic equations: the former determine the visible movements of bodies of ordinary size, the latter the minute occurrences in the smallest parts.
We may say, speaking somewhat roughly, that a stimulus applied to the nervous system, like a spark to dynamite, is able to take advantage of the stored energy in unstable equilibrium, and thus to produce movements out of proportion to the proximate cause.
Mechanical movements are of no interest to the psychologist, and it has only been necessary to define them in order to be able to exclude them.
The next point is to distinguish between movements that are instinctive and movements that are acquired by experience.
To take extreme cases, every animal at birth can take food by instinct, before it has had opportunity to learn; on the other hand, no one can ride a bicycle by instinct, though, after learning, the necessary movements become just as automatic as if they were instinctive.
Next day you repeat the experiment, and you find that the cat gets out much more quickly than the first time, although it still makes some random movements.
Peckham have shown that the sting of the wasp is NOT UNERRING, as Fabre alleges, that the number of stings is NOT CONSTANT, that sometimes the caterpillar is NOT PARALYZED, and sometimes it is KILLED OUTRIGHT, and that THE DIFFERENT CIRCUMSTANCES DO NOT APPARENTLY MAKE ANY DIFFERENCE TO THE LARVA, which is not injured by slight movements of the caterpillar, nor by consuming food decomposed rather than fresh caterpillar.
4) That instinct supplies the impulses to experimental movements which are required for the process of learning;