tions of the
mind, looked at in themselves, do not contain error. The mind
does not err in the mere act of imagining, but only in so far as
it is regarded as being without the idea, which excludes the
existence of such things as it imagines to be present to it. If
the mind, while imagining non--existent things as present to it,
is at the same time conscious that they do not really exist, this
power of imagination must be set down to the efficacy of its
nature, and not to a fault, especially if this faculty of
imagination depend solely on its own nature--that is (I. Def.
vii.), if this faculty of imagination be free.

PROP. XVIII. If the human body has once been affected by two or
more bodies at the same time, when the mind afterwards imagines
any of them, it will straightway remember the others also.

Proof.--The mind (II. xvii. Coroll.) imagines any given body,
because the human body is affected and disposed by the
impressions from an external body, in the same manner as it is
affected when certain of its parts are acted on by the said
external body; but (by our hypothesis) the body was then so
disposed, that the mind imagined two bodies at once; therefore,
it will also in the second case imagine two bodies at once, and
the mind, when it imagines one, will straightway remember the
other. Q.E.D.

Note.--We now clearly see what Memory is. It is simply a
certain association of ideas involving the nature of things
outside the human body, which association arises in the mind
according to the order and association of the modifications
(affectiones) of the human body. I say, first, it is an
association of those ideas only, which involve the nature of
things outside the human body: not of ideas which answer to the
nature of the said things: ideas of the modifications of the
human body are, strictly speaking (II. xvi.), those which involve
the nature both of the human body and of external bodies. I say,
secondly, that this association arises according to the order and
association of the modifications of the human body, in order to
distinguish it from that association of ideas, which arises from
the order of the intellect, whereby the mind perceives things
through their primary causes, and which is in all men the same.
And hence we can further clearly understand, why the mind from
the thought of one thing, should straightway arrive at the
thought of another thing, which has no similarity with the first;
for instanc