ne another, when they
cause some change suitable to Extension; and Spirits to act upon one
another, when they cause some change suitable to a Thought; he infers, that
when a Body acts upon a Spirit, that cannot be by causing any change of
motion, of figure, or parts, as having none of all these; nor when a Spirit
acts upon a Body, that cannot be by producing any change of Thought, as
having none: But, when this Body, or its motion, or figure, or other thing,
depending upon its nature, can be perceived by a Spirit, so as, upon that
occasion, this Spirit has thoughts, it had not before, it may be said, that
the Body has acted upon this Spirit, for as much as it has caused all the
change in it, whereof it was capable according to its nature.

In the _Sixth_, After he hath shew'd, what is to be understood by what we
call _Soul_, and by what we call _Body_, he labours to make it out, that we
are much more assured of the Existence of the Soul, than of that of the
Body, which he conceives he can prove from hence, that we cannot doubt,
that we think, because even doubting is thinking; but one may doubt,
whether one has a body, for several reasons, which he alledges, and thinks
so cogent, that he concludes, it is not evident to him by the light of
reason, that he has a Body. But supposing, there be Bodies, he examines,
what are the Operations, that belong to the Soul, and what those, that
belong to the Body; and lastly, what those, that result from the Union of
both: And then explains, how all those operations are perform'd, and
particularly, _Sensation_; where he shews, that the Nerves, holding at one
end to the Brain, whereof they are but Allongations, and being at the other
end extended to the extremities of the Body; when an Object comes to touch
those exterior ends of the Nerves, the interior ones in the Brain are
presently shaken; and cause different sensations according to the
diversitie of Nerves, and the differing manner, in which they are shaken.
And to shew, that 'tis this shaking, that causes Sensation, he notes, that
if any thing shakes the interior parts of the Nerves, though the object be
absent, the Soul has presently the same {310} sensations, as it would have,
if it were present. As, if one should knock on's head forcibly against a
wall, the shaking, which the blow gives to the Brain, moving the interior
extremities of the Nerve, which causes the sensation of Light, the Soul has
the same sensation, which it would