Earlier we defined sensibility as the faculty by which we experience pleasure and pain. But what are pleasure and pain? We can't give a perfect answer to this question, but we can seek out their essential characteristics and try to identify their causes.

These states of consciousness have three main characteristics:

1. Pleasure and pain are affective phenomena – we experience them without having to do anything at all. Where pleasure and pain are concerned, we're passive. Of course, there's no complete passivity in psychological life – we certainly react in order to decrease pain or increase pleasure. Nevertheless, when it comes to pleasure and pain, we're mostly passive.

2. The second characteristic of these phenomena is their necessity. We can't stop ourselves from experiencing pleasure and pain. They're the necessary consequence of the prior event that brought them about, and the only way we can modify them is by modifying that event. Through an exercise of will, of course, we can avert our consciousness from pleasure and pain, or make them more intense by giving them our attention. By doing so, we can even take some pleasure in pain, as when we feel melancholy. But while we can have some influence over these feelings, we never control them completely. This was the illusion of the Stoics and the Epicureans, who thought they could eliminate pain by sheer willpower.

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