P4: It is impossible that a man should not be a part of Nature, and that he should be able to undergo no changes except those which can be understood through his own nature alone, and of which he is the adequate cause.

Dem.: [i] The power by which singular things (and consequently, {any} man) preserve their being is the power itself of God/Nature, or Nature (by IP24 C), not insofar as it is infinite, but insofar as it can be explained through the man’s actual essence (by IIIP7). The man’s power, therefore, insofar as it is explained through his actual essence, is part of God/Nature or Nature’s infinite power, i.e. (by IP34), of its essence. This was the first point.

[ii] Next, if it were possible that a man could undergo no changes except those which can be understood through the man’s nature alone, it would follow (by IIIP4 and P6) that he could not perish, but that necessarily he would always exist. And this would have to follow from a cause whose power would be either finite or infinite, viz. either from the power of the man alone, who would be able to avert from himself other changes which could arise from external causes, or from the infinite power of Nature, by which all singular things would be directed so that the man could undergo no other changes except those which assist his preservation.

But the first is absurd (by P3, whose demonstration is universal and can be applied to all singular things).

Therefore, if it were possible for a man to undergo no changes except those which could be understood through the man’s nature alone, so that (as we have already shown) he would necessarily always exist, this would have to follow from God/Nature’s infinite power; and consequently (by IP 16) the order of the whole of Nature, insofar as it is conceived under the attributes of Extension and Thought, would have to be deduced from the necessity of the divine/universal nature, insofar as it is considered to be affected with the idea of some man. And so (by IP21) it would follow that the man would be infinite. But this (by part [i] of this demonstration) is absurd

Therefore, it is impossible that a man should undergo no other changes except those of which he himself is the adequate cause, q.e.d.

Cor.: From this it follows that man is necessarily always subject to passions, that he follows and obeys the common order of Nature, and accommodates himself to it as much as the nature of things requires.