P38: Those things which are common to all, and which are equally in the part and in the whole, can only be conceived adequately.

Dem.: Let A be something which is common to all bodies, and which is equally in the part of each body and in the whole. I say that A can only be conceived adequately. For its idea (by P7 C) will necessarily be adequate in God/Nature, both insofar as he has the idea of the human Body and insofar as he has ideas of its affections, which (by P16, P25, and P27) involve in part both the nature of the human Body and that of external bodies. That is (by P12 and P13), this idea will necessarily be adequate in God/Nature insofar as he constitutes the human Mind, or insofar as he has ideas that are in the human Mind. The Mind therefore (by P11 C) necessarily perceives A adequately, and does so both insofar as it perceives itself and insofar as it perceives its own or any external body. Nor can A be conceived in another way, q.e.d.

Cor.: From this it follows that there are certain ideas, or notions, common to all men. For (by L2) all bodies agree in certain things, which (by P38) must be perceived adequately, or clearly and distinctly, by all.