Now we'll examine certain states – simultaneously physiological and psychological – connected to the three faculties of conception. Their common trait is that images become so vivid within them that they are mistaken for perceptions.

Consider dreaming, the most common of these states. The physiological conditions of dreaming are poorly understood, so we'll leave these aside and simply try to understand the relationship between dreams and psychic activity. Some philosophers claim that, during sleep, all thought ceases. We've already touched on this in our discussion of the association of ideas, where we decided that the chain of ideas is uninterrupted and that – even during sleep – we have sensations that give us ideas. We also argued that the self is entirely conscious, so that – if thought were to cease – consciousness would vanish and the self would cease to act or exist. But how could the self be reborn again and again after having been destroyed in this way? The answer is that it couldn't. So even in sleep, the soul never entirely sleeps and is never destroyed. In fact, according to Jouffroy, there is no psychological sleep, for the soul never sleeps at all. Neither is sleep, according to him, a purely physical phenomenon. For support, he cites our “indifference to habitual sounds” during sleep as well as the capacity some have for dreaming at will. These facts can be explained only if the self never sleeps in any absolute sense.