ee. But, before I go further, I
should like here to explain at greater length Prop. x of this
part, in order that we may clearly understand how one idea is
contrary to another. In the note to II. xvii. we showed that the
idea, which constitutes the essence of mind, involves the
existence of body, so long as the body itself exists. Again, it
follows from what we pointed out in the Corollary to II. viii.,
that the present existence of our mind depends solely on the
fact, that the mind involves the actual existence of the body.
Lastly, we showed (II. xvii., xviii. and note) that the power of
the mind, whereby it imagines and remembers things, also depends
on the fact, that it involves the actual existence of the body.
Whence it follows, that the present existence of the mind and its
power of imagining are removed, as soon as the mind ceases to
affirm the present existence of the body. Now the cause, why the
mind ceases to affirm this existence of the body, cannot be the
mind itself (III. iv.), nor again the fact that the body ceases
to exist. For (by II. vi.) the cause, why the mind affirms the
existence of the body, is not that the body began to exist;
therefore, for the same reason, it does not cease to affirm the
existence of the body, because the body ceases to exist; but
(II. xvii.) this result follows from another idea, which excludes
the present existence of our body and, consequently, of our mind,
and which is therefore contrary to the idea constituting the
essence of our mind.

PROP. XII. The mind, as far as it can, endeavours to conceive
those things, which increase or help the power of activity in the
body.

Proof.--So long as the human body is affected in a mode, which
involves the nature of any external body, the human mind will
regard that external body as present (II. xvii.), and
consequently (II. vii.), so long as the human mind regards an
external body as present, that is (II. xvii. note), conceives it,
the human body is affected in a mode, which involves the nature
of the said external body; thus so long as the mind conceives
things, which increase or help the power of activity in our body,
the body is affected in modes which increase or help its power of
activity (III. Post. i.); consequently (III. xi.) the mind's
power of thinking is for that period increased or helped. Thus
(III. vi., ix.) the mind, as far as it can, endeavours to imagine
such things. Q.E.D.

PROP. XIII. When the mind c