Selection from – Ethics – Part III. On the Nature and Origin of the Emotions (Page 21)

Spinoza's Words: (pleasure is a transition toward perfection)

Definitions

II. Pleasure is the transition of a man from a less to a greater perfection.

III. Pain is the transition of a man from a greater to a less perfection.

Explanation—I say transition: for pleasure is not perfection itself. For, if man were born with the perfection to which he passes, he would possess the same, without the emotion of pleasure. This appears more clearly from the consideration of the contrary emotion, pain. No one can deny, that pain consists in the transition to a less perfection...

Neither can we say, that pain consists in the absence of a greater perfection. For absence is nothing, whereas the emotion of pain is an activity; wherefore this activity can only be the activity of transition from a greater to a less perfection—in other words, it is an activity whereby a man's power of action is lessened or constrained.