P17: If we imagine that a thing which usually affects us with an affect of Sadness is like another which usually affects us with an equally great affect of Joy, we shall hate it and at the same time love it.

Dem.: For (by hypothesis) this thing is through itself the cause of Sadness, and (by P13 S) insofar as we imagine it with this affect, we hate it. And moreover, insofar as it has some likeness to the other thing, which usually affects us with an equally great affect of Joy, we shall love it with an equally great striving of Joy (by P16). And so we shall both hate it and at the same time love it, q.e.d.

Schol.: This constitution of the Mind which arises from two contrary affects is called vacillation of mind, which is therefore related to the affect as doubt is to the imagination (see IIP44 S); nor do vacillation of mind and doubt differ from one another except in degree.

But it should be noted that in the preceding Proposition I have deduced these vacillations of mind from causes which are the cause through themselves of one affect and the accidental cause of the other. I have done this because in this way they could more easily be deduced from what has gone before, not because I deny that vacillations of mind for the most part arise from an object which is the efficient cause of each affect. For the human Body (by IIPost. 1) is composed of a great many individuals of different natures, and so (by IIA1″ ), it can be affected in a great many different ways by one and the same body. And on the other hand, because one and the same thing can be affected in many ways, it will also be able to affect one and the same part of the body in many different ways. From this we can easily conceive that one and the same object can be the cause of many and contrary affects.