Memory is the faculty whereby a past state of consciousness is reproduced within us so that we recognize it as past. A memory must fulfill both of these conditions.

This definition shows that there's something inexact about the expression “I remember such and such a thing.” Strictly speaking, we remember not things but only the states of consciousness in which they first appeared. This is what Royer-Collard means when he says that, in fact, we only remember ourselves.

Memory can take different forms. Sometimes it's quick, so that seeing something once is enough to remember it, and sometimes it's easy, so we can recall things without difficulty. Sometimes it's exact, recalling things with precision, and sometimes it's tenacious, retaining a state of consciousness for an extended period of time.

It's rare to find all these qualities together in the same individual. But memory can be classified still further. We have memories, for example, of verses, colors, sounds, numbers, and so on. The general character of a man's mind often can be deduced from the type of memory he has.

Means for improving memory have often been sought, and together these efforts compose mnemonics. Although poorly developed, this science has come across some useful principles that might have been deduced from the definition of memory itself. The more of ourselves we include in a memory, the easier it is for us to remember it, for states of consciousness in which we're active are easier to retain or reproduce than others.