It has a definite duration in time, and an inevitable moment comes in which it must die.
In the great problem of psychology we cannot expect to reach a scientific result, if we persist in restricting ourselves to the contemplation of one fact.

We must avail ourselves of all accessible facts.

Human psychology can never be completely resolved except through comparative psychology.

With Descartes, we must inquire whether the souls of animals be relations of the human soul, less perfect members in the same series of development.

We must take account of what we discover in the intelligent principle of the ant, as well as what we discern in the intelligent principle of man.