Christopher asked:

If you accept the existence of the spirit, then how can sleep be explained? For an immortal spirit
would not sleep, as it not an organism in the sense of an animal. If our spirit existed as part of our
self, we should therefore have a form of consciousness in our sleep.

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The traditional dualist thinks that we have a spirit and a body. One easy explanation of why we sleep
is that it is a bodily function separate to the goings on of the spirit. Although the spirit may be immortal
the body is not. It is finite and has limits. Sleep could be for some regenerative purpose.

The question of our continued consciousness during sleep is more interesting. Perhaps the spirit can
only operate through a fully functioning body, some complex interaction may be necessary between
the two that fails to work in sleep or perhaps dreams are the awareness of the spirit's goings on.

In fact how do we know that the spirit is immortal or that it can dispose of rest? after all, immortality
does not mean ever-alert, perhaps the spirit too takes time out when the body sleeps. Given the
assumption that we do have a spirit, the fact that we do not have a full consciousness in sleep
supports the idea that the spirit does itself need rest.

But even if we do not have a spirit the question of the purpose of sleep and dreams is just as
interesting. In biological terms sleep must have some enormous advantage, given that it leaves us
defenceless for hours a day. What can it be?

We know that the body is able to make essential proteins at a much faster rate while sleeping than
when awake, and without sleep the body cannot store energy as efficiently, and without any sleep we
would eventually die.

Brian Tee
Dept of Philosophy
University of Sheffield.

Second opinion:Although Christopher does not make this explicit, there does seem to be a problem
with accounting for a sleeping spirit, or soul, on Descartes' dualist theory of 'mental substance' and
'material substance'.

According to Descartes, material substance is defined by the essential attributeof being extended
and occupying a position in space. In a similar way, mental substance is defined by the essential
attributeof thought, or consciousness. In these terms, it would seem that the idea of a mind that was
not conscious for some period of time would be no less absurd than the idea of a material body that
was not spatially extended, or did not occupy a spatial position for some period of time.

The solution is to accept that the spirit or soul can never be completely lacking in consciousness.
Even in a dreamless sleep, therefore, a defender of Descartes would have to say that there is some
small degree consciousness there. Descartes is only committed to the claim that 'I exist is true
whenever I think it'. He is not committed to holding that whenever I exist, I must be consciously aware
that I am having thoughts.

Geoffrey Klempner