P26: What we strive for from reason is nothing but understanding; nor does the Mind, insofar as it uses reason, judge anything else useful to itself except what leads to understanding.

Dem.: The striving to preserve itself is nothing but the essence of the thing itself (by IIIP7), which, insofar as it exists as it does, is conceived to have a force for persevering in existing (by IIIP6) and for doing those things that necessarily follow from its given nature (see the Definition of Appetite in IIIP9 S). But the essence of reason is nothing but our Mind, insofar as it understands clearly and distinctly (see the Definition of this in IIP40 S2). Therefore (by IIP40) whatever we strive for from reason is nothing but understanding.

Next, since this striving of the Mind, by which the Mind, insofar as it reasons, strives to preserve its being, is nothing but understanding (by the first part of this demonstration), this striving for understanding (by P22 C) is the first and only foundation of virtue, nor do we strive to understand things for the sake of some end (by P25). On the contrary, the Mind, insofar as it reasons, cannot conceive anything to be good for itself except what leads to understanding (by D1), q.e.d.