[27] This proposition is self-evident, for the definition of any given thing affirms and does not deny the existence of the thing; that is to say, it posits the essence of the thing and does not negate it. So long, therefore, as we attend only to the thing itself, and not to external causes, we shall discover nothing in it which can destroy it.
CHAPTER XIII
THE PSYCHOLOGY OF THE EMOTIONS
The Association of the Emotions
If the human body has at any time been simultaneously affected by two bodies, whenever the mind afterwards imagines one of them, it will immediately remember the other. But the imaginations of the mind indicate rather the modifications of our body than the nature of external bodies, and therefore if the body, and consequently the mind, has been at any time, simultaneously affected by two emotions, whenever it is afterwards affected by one of them, it will also be affected by the other.
Let the mind be supposed to be affected at the same time by two emotions, its power of action not being increased or diminished by one, while it is increased or diminished by the other. From the preceding proposition it is plain that when the mind is afterwards affected by the first emotion through its true cause, which (by hypothesis) of itself neither increases nor diminishes the mind's power of thinking, it will at the same time be affected by the other emotion, which does increase or diminish that power, that is to say, it will be affected with joy or sorrow; and thus the thing itself will be the cause of joy or of sorrow, not of itself, but accidentally. In the same way it can easily be shown that the same thing may accidentally be the cause of desire.
The fact that we have contemplated a thing with an emotion of joy or sorrow, of which it is not the efficient cause, is a sufficient reason for being able to love or hate it.
We now understand why we love or hate certain things from no cause which is known to us, but merely from sympathy or antipathy, as they say. To this class, too, are to be referred those objects which affect us with joy or sorrow solely because they are somewhat like objects which usually affect us with those emotions. I know indeed that the writers who first introduced the words "Sympathy" and "Antipathy" desired thereby to signify certain hidden qualities of things, but nevertheless I believe that we shall be permitted to understand by those names qualities which are plain and well known.