our body's power of
action, the idea of that thing increases, diminishes, helps, or limits
our mind's power of thought.

We thus see that the mind can suffer great changes, and can pass now to
a greater and now to a lesser perfection; these passions explaining to
us the emotions of joy and sorrow. By _joy_, therefore, in what
follows, I shall understand the passion by which the mind passes to a
greater perfection; by _sorrow_, on the other hand, the passion by which
it passes to a less perfection. The emotion of joy, related at the same
time both to the mind and the body, I call _pleasurable excitement_
(_titillatio_) or _cheerfulness_; that of sorrow I call _pain_ or
_melancholy_. It is, however, to be observed that pleasurable excitement
and pain are related to a man when one of his parts is affected more
than the others; cheerfulness and melancholy, on the other hand, when
all parts are equally affected. What the nature of desire is I have
explained; and besides these three--joy, sorrow, and desire--I know of
no other primary emotion, the others springing from these.

_Definitions of the Principal Emotions_

I.--_Desire_ is the essence itself of man in so far as it is conceived
as determined to any action by any one of his modifications.

_Explanation._--We have said above, that desire is appetite which is
self-conscious, and that appetite is the essence itself of man in so far
as it is determined to such acts as contribute to his preservation. But
I have taken care to remark that in truth I cannot recognize any
difference between human appetite and desire. For whether a man be
conscious of his appetite or not, it remains one and the same appetite,
and so, lest I might appear to be guilty of tautology, I have not
explained desire by appetite, but have tried to give such a definition
of desire as would include all the efforts of human nature to which we
give the name of appetite, desire, will, or impulse. For I might have
said that desire is the essence itself of man in so far as it is
considered as determined to any action; but from this definition it
would not follow that the mind could be conscious of its desire or
appetite, and therefore, in order that I might include the cause of this
consciousness, it was necessary to add the words, _in so far as it is
conceived as determined to any action by any one of his modifications_.
For by a modification of the human essence we understand any
constitution of that e