Similarly, when we
desire to drink, a certain dryness of the throat is produced
which moves its nerves, and by their means the internal
portions of the brain; and this movement causes in the mind
the sensation of thirst, because in this case there is nothing
more useful to us than to become aware that we have need to
drink for the conservation o our health; and the same holds
good in other instances.
From this it is quite clear that, notwithstanding the
supreme goodness of God, the nature of man, inasmuch as it is
composed of mind and body, cannot be otherwise than sometimes
a source of deception. For if there is any cause which
excites, not in the foot but in some part of the nerves which
are extended between the foot and the brain, or even in the
brain itself, the same movement which usually is produced when
the foot is detrimentally affected, pain will be experienced
as though it were in the foot, and the sense will thus
naturally be deceived; for since the same movement in the
brain is capable of causing but one sensation in the mind, and
this sensation is much more frequently excited by a cause
which hurts the foot than by another existing in some other
quarter, it is reasonable that it should convey to the mind
pain in the foot rather than in any other part of the body.
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