Hippocratic doctors discussed two forms of total loss of consciousness. The most common one, where they equated fainting with a separation of the soul, and another form - independent from the psuchê - where they saw the concurrence of numerous independent bodily symptoms that ended up in a swoon. The momentary disruption of cognitive functions that occurred during fainting, and their definite cessation with death made them conceive the soul as both a broad notion that subsumed numerous mental capacities (which transiently separated from the body during swoons) and as a life force that abandoned the body for good with death.

Some Hippocratic doctors regarded sleep as a healthy process, and some as a pathological one; some of them struggled to distinguish between hallucinations and nightmares, and some between deep dreamless sleep and total loss of consciousness. This chapter explores how different treatises from the Hippocratic corpus navigated these ambiguities, how they explained different depth of sleep (i.e. different levels of consciousness), and how such understanding relates to their views on mental capacities (which they subsumed in concepts such as phronesis, sunesis, gnômê, and nous).