Little Question Two of Question Two
On universal and partial sleep
It is asked what universal and partial sleep are.
To this it is answered that sleep is called universal when the exterior senses together with the intellective power are lulled and rest.
Or universal sleep, according to Avicenna in the first book, chapter 3, De subeth, is the return of the animal spirit from the instruments of the senses and its motion toward its own principle, namely the brain. With this return, the instruments themselves are deprived of the outgoing operation in those instruments, except for those instruments which are necessary in order that life may remain. These are, as it were, instruments of the spirit, such as the lung, heart, arteries, and diaphragm.
Partial sleep, however, is threefold: either when the exterior and interior senses are lulled while the intellective power is awake; or when the exterior senses are bound while the interior senses are released from sleep, as is evident in those who walk by night while sleeping; or when the interior senses are bound while the exterior senses are released.
According to certain persons, the first of these is possible. They prove it thus: just as the interior sense can be awake when the exterior sense is bound by sleep, as will be evident below, so nothing seems to prevent an intellect which has been sufficiently habituated by habits and sensations from often being awake—that is, cognitively actualized with respect to intelligible things—while both sensitive powers are lulled.
But if it is said, according to the philosophers, that in order actually to understand it is necessary to contemplate phantasms, and therefore the intellect is actualized only consequently upon the actualization of the phantasy, it is answered that, if that common statement of the philosophers possesses truth, it is understood of the common course of human cognition and of the first acquisition of intellectual knowledge.
Or it is true with respect to those intelligible things concerning which the intellect does not yet possess within itself sufficiently intrinsic habits and species according to which it can go forth into act without the assistance of the sensitive powers.
The case is otherwise, however, with an intellect thus habituated. In order to actualize itself, it is not necessary that it turn itself toward phantasms.
The person who said, “I sleep, and my heart is awake,” appears to have experienced this intellectual wakefulness while the senses were bound by sleep. According to this opinion, the sleep of both kinds of senses cannot lull the intellect unless it is bare or insufficiently habituated.
The second condition—namely, the binding of the exterior senses while the interior senses are released from sleep—is also manifestly apparent in certain persons who rise and walk while sleeping at night without stumbling, and perform certain works of those who are awake. Namely, they speak, shout, dress themselves, climb trees, leave their houses for fields and woods, pursue their enemies and sometimes kill them, and return to their beds, as Albert says in his commentary upon the book On Sleep and Waking.
Third, some persons can also be awake according to the exterior sensitive power and asleep according to the interior, as I heard concerning one such person who actually perceived.
Such persons are also sometimes awake according to sense and at the same time according to the intellective power. The latter can perhaps be actualized in some manner upon the actualization of the exterior senses while the interior senses are suspended from every act.
Hence, if in a waking human being God were for a time to suspend every actualization of the interior senses without impeding the influx of power and spirits to the exterior senses, what would prevent the human being both from seeing sensitively and reasoning intellectually about the things seen, and thus sufficiently directing and guarding himself in walking or climbing?
These things are from Henry of Hesse in the place cited above.