Q.E.D.
PROP.XI.

Whatsoever increases or diminishes, helps or hinders the power of activity in our body, the idea thereof increases or diminishes, helps or hinders the power of thought in our mind.
Proof .-- This proposition is evident from II.vii.or from II.
xiv.
Note .-- Thus we see, that the mind can undergo many changes, and can pass sometimes to a state of greater perfection, sometimes to a state of lesser perfection.

These passive states of transition explain to us the emotions of pleasure and pain.
By pleasure therefore in the following propositions I shall signify a passive state wherein the mind passes to a greater perfection.

By pain I shall signify a passive state wherein the mind passes to a lesser perfection.

Further, the emotion of pleasure in reference to the body and mind together I shall call stimulation (titillatio) or merriment (hilaritas), the emotion of pain in the same relation I shall call suffering or melancholy.
But we must bear in mind, that stimulation and suffering are attributed to man, when one part of his nature is more affected than the rest, merriment and melancholy, when all parts are alike affected.