CHAPTER IV.
THE CLASSIFICATION AND THE NOMENCLATURE OF MENTAL OPERATIONS.
If, as has been set forth in the preceding chapter, all mental states are effects of physical causes, it follows that what are called mental faculties and operations are, properly speaking, cerebral functions, allotted to definite, though not yet precisely assignable, parts of the brain.
These functions appear to be reducible to three groups, namely: Sensation, Correlation, and Ideation.
The organs of the functions of sensation and correlation are those portions of the cerebral substance, the molecular changes of which give rise to impressions of sensation and impressions of relation.
The changes in the nervous matter which bring about the effects which we call its functions, follow upon some kind of stimulus, and rapidly reaching their maximum, as rapidly die away.

The effect of the irritation of a nerve-fibre on the cerebral substance with which it is connected may be compared to the pulling of a long bell-wire.

The impulse takes a little time to reach the bell; the bell rings and then becomes quiescent, until another pull is given.

So, in the brain, every sensation is the ring of a cerebral particle, the effect of a momentary impulse sent along a nerve-fibre.
If there were a complete likeness between the two terms of this very rough and ready comparison, it is obvious that there could be no such thing as memory.

A bell records no audible sign of having been rung five minutes ago, and the activity of a sensigenous cerebral particle might similarly leave no trace.