P3: The actions of the Mind arise from adequate ideas alone; the passions depend on inadequate ideas alone.

Dem.: The first thing that constitutes the essence of the Mind is nothing but the idea of an actually existing Body (by IIP11 and P13); this idea (by IIP15) is composed of many others, of which some are adequate (IIP38 C), and others inadequate (by IIP29 C).

Therefore, whatever follows from the nature of the Mind and has the Mind as its proximate cause, through which it must be understood, must necessarily follow from an adequate idea or an inadequate one. But insofar as the Mind has inadequate ideas (by P1), it necessarily is acted on. Therefore, the actions of the Mind follow from adequate ideas alone; hence, the Mind is acted on only because it has inadequate ideas, q.e.d.

Schol.: We see, then, that the passions are not related to the Mind except insofar as it has something which involves a negation, or insofar as it is considered as a part of nature which cannot be perceived clearly and distinctly through itself, without the others. In this way I could show that the passions are related to singular things in the same way as to the Mind, and cannot be perceived in any other way. But my purpose is only to treat of the human Mind.