straightway be affected by one or the other emotion, and
consequently the thing, which we perceive to have the same point
of resemblance, will be accidentally (III. xv.) a cause of
pleasure or pain. Thus (by the foregoing Corollary), although
the point in which the two objects resemble one another be not
the efficient cause of the emotion, we shall still regard the
first--named object with love or hate. Q.E.D.

PROP. XVII. If we conceive that a thing, which is wont to affect
us painfully, has any point of resemblance with another thing
which is wont to affect us with an equally strong emotion of
pleasure, we shall hate the first--named thing, and at the same
time we shall love it.

Proof.--The given thing is (by hypothesis) in itself a cause
of pain, and (III. xiii. note), in so far as we imagine it with
this emotion, we shall hate it: further, inasmuch as we conceive
that it has some point of resemblance to something else, which is
wont to affect us with an equally strong emotion of pleasure, we
shall with an equally strong impulse of pleasure love it
(III. xvi.); thus we shall both hate and love the same thing.
Q.E.D.

Note.--This disposition of the mind, which arises from two
contrary emotions, is called vacillation; it stands to the
emotions in the same relation as doubt does to the imagination
(II. xliv. note); vacillation and doubt do not differ one from
the other, except as greater differs from less. But we must bear
in mind that I have deduced this vacillation from causes, which
give rise through themselves to one of the emotions, and to the
other accidentally. I have done this, in order that they might
be more easily deduced from what went before; but I do not deny
that vacillation of the disposition generally arises from an
object, which is the efficient cause of both emotions. The human
body is composed (II. Post. i.) of a variety of individual parts
of different nature, and may therefore (Ax.i. after Lemma iii.
after II. xiii.) be affected in a variety of different ways by
one and the same body; and contrariwise, as one and the same
thing can be affected in many ways, it can also in many different
ways affect one and the same part of the body. Hence we can
easily conceive, that one and the same object may be the cause of
many and conflicting emotions.

PROP. XVIII. A man is as much affected pleasurably or painfully
by the image of a thing past or future as by the image of a thing
present.

P