They also introduce the element of time into the action of the nervous mechanism.

An impression, which without them might have forthwith ended in reflex action, is delayed, and with this duration come all those important effects arising through the interaction of many impressions, old and new, upon each other.
There is no such thing as a spontaneous, or self-originated, thought.
Every intellectual act is the consequence of some preceding act.

It comes into existence in virtue of something that has gone before.

Two minds constituted precisely alike, and placed under the influence of precisely the same environment, must give rise to precisely the same thought.

To such sameness of action we allude in the popular expression "common-sense"-- a term full of meaning.