4. In the Fourth, he teaches, that though Experience seems to evince, that the Soul moves the Body, and that one Body moves an other; yet there is nothing but God, that can produce any notion in the World, and all other Agents, which we believe to be the Cause of this or that Motion, are no more but the Occasion thereof. In doing this, he advances certain Axioms, and Conclusions, which are in short,
a. The Axioms: That no substance has that of it self, which it can loose, without ceasing to be, what it is: That every body may loose of its motion, till it have no more left, without ceasing to be a Body: That we cannot conceive but two sorts of substances, vid. a Spirit (or That which thinketh) and a Body, wherefore they must be considered as the Causes of all, that happens, and what cannot proceed from the one, must necessarily be adscribed to the other: That to Move, or to cause motion, is an Action: That an Action cannot be continued but by the Agent, who began it.
b. The Conclusions: That no Body hath Motion of it self: That the First Mover of Bodies not a Body: That it cannot be but a Spirit, that is the First Mover: That it cannot be but the same Spirit, who has begun to move Bodies, that continues to move.
In the Fifth, He treats of the Union of the Body and Soul, and the manner, how they act one upon the other; and esteems it not more difficult to conceive the Action of Spirits upon Bodies, and of Bodies upon Spirits, than to conceive the Action of Bodies upon Bodies: the cause of the great difficulty in understanding the two former, arising (according to him) from thence, that we will conceive the one by the other, not considering, that every thing acting according to its own nature, we shall never know the action of one Agent, if we will examine it by the notions we have of another, that is of a quite differing nature. Here he notes, that the Action of Bodies upon Bodies is not
more known to us; than that of Spirits upon Bodies, or of Bodies upon Spirits; and yet most men admire nothing but this, believing to know the other; whereas he Judges, that all things being well examin'd, the Action of Bodies upon Bodies is no more conceivable, than that of Spirits upon Bodies. Mean while the opinion of the Authour touching this subject, is, That the union of Soul and Body consists onely in this, that certain motions of the Body are followed by certain Cogitations of the Soul, and, on the contrary, that certain Thoughts of the Soul are follow'd by certain Motions of the Body. And, having supposed, that Bodies are said to act upon one 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
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 have, if it saw a thousand Candles: On the contrary, if the interior extremities of the nerves are not shaken, though the object be present, it causes no sensation; whence it comes, that if a strong Ligature be made upon the middle of the Arm, and the hand be then prickt, no pain is felt, because the shaking of the nerves that are pricked, being stopped by the Ligature, cannot reach to the extremities of the Nerves, that are within the Brain.
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The following Errata, left by the Press in Num. 16, the Reader is desired thus to correct.