The mind must assimilate its new material as much as possible, thus making the old stand for the new.
Otherwise there would be no containing the fragmentary details which we should have to remember and handle.

Furthermore, it is through this tendency that we go on to form the great classes of objects--such as man, animal, virtue--into which numbers of similar details are put, and which we call General Notions or Concepts.
We may understand by Assimilation, therefore, the general tendency of new experiences to be treated by us in the ways which similar material has been treated before, with the result that the mind proceeds from the particular case to the general class.
Summing up our outcome so far, we find that general psychology has reached three great principles in its investigation of knowledge.
First, we have the combining tendency of the mind, the grouping together and relating of mental states and of things, called _Apperception_.

Then, second, there are the particular relations established among the various states, etc., which are combined; these are called _Associations_ of Ideas.

And, third, there is the tendency of the mind to use its old experiences and habits as general patterns or nets for the sorting out and distributing of all the new details of daily life; this is called _Assimilation_.
II.

Let us now turn to the second great aspect of the mind, as general or introspective psychology considers it, the aspect which presents itself in Action or conduct.