P30: We can have only an entirely inadequate knowledge of the duration of our Body.

Dem.: Our body’s duration depends neither on its essence (by A1), nor even on God/Nature’s absolute nature (by IP21). But (by IP28) it is determined to exist and produce an effect from such {NS: other} causes as are also determined by others to exist and produce an effect in a certain and determinate manner, and these again by others, and so to infinity. Therefore, the duration of our Body depends on the common order of nature and the constitution of things.

But adequate knowledge of how things are constituted is in God/Nature, insofar as he has the ideas of all of them, and not insofar as he has only the idea of the human Body (by P9 C). So the knowledge of the duration of our Body is quite inadequate in God/Nature, insofar as he is considered to constitute only the nature of the human Mind, i.e. (by P11 C ), this knowledge is quite inadequate in our Mind, q.e.d.