P29: The idea of the idea of any affection of the human Body does not involve adequate knowledge of the human Mind.

Dem.: For the idea of an affection of the human Body (by P27) does not involve adequate knowledge of the Body itself, or does not express its nature adequately, i.e. (by P13), does not agree adequately with the nature of the Mind; and so (by IA6) the idea of this idea does not express the nature of the human mind adequately, or does not involve adequate knowledge of it, q.e.d.

Cor.: From this it follows that so long as the human Mind perceives things from the common order of nature, it does not have an adequate, but only a confused and mutilated knowledge of itself, of its own Body, and of external bodies. For the Mind does not know itself except insofar as it perceives ideas of the affections of the body (by P23). But it does not perceive its own Body (by P19) except through the very ideas themselves of the affections {of the body}, and it is also through them alone that it perceives external bodies (by P26). And so, insofar as it has these {ideas}, then neither of itself (by P29), nor of its own Body (by P27), nor of external bodies (by P25) does it have an adequate knowledge, but only (by P28 and P28S) a mutilated and confused knowledge, q.e.d.

Schol.: I say expressly that the Mind has, not an adequate, but only a confused {NS: and mutilated} knowledge, of itself, of its own Body, and of external bodies, so long as it perceives things from the common order of nature, i.e., so long as it is determined externally, from fortuitous encounters with things, to regard this or that, and not so long as it is determined internally, from the fact that it regards a number of things at once, to understand their agreements, differences, and oppositions. For so often as it is disposed internally, in this or another way, then it regards things clearly and distinctly, as I shall show below.