nothing
else but to act according to the laws of one's own nature. But
we only act, in so far as we understand (III. iii.): therefore
to act in obedience to virtue is in us nothing else but to act,
to live, or to preserve one's being in obedience to reason, and
that on the basis of seeking what is useful for us (IV. xxii.
Coroll.). Q.E.D.

PROP. XXV. No one wishes to preserve his being for the sake of
anything else.

Proof.--The endeavour, wherewith everything endeavours to
persist in its being, is defined solely by the essence of the
thing itself (III. vii.); from this alone, and not from the
essence of anything else, it necessarily follows (III. vi.) that
everyone endeavours to preserve his being. Moreover, this
proposition is plain from IV. xxii. Coroll., for if a man should
endeavour to preserve his being for the sake of anything else,
the last--named thing would obviously be the basis of virtue,
which, by the foregoing corollary, is absurd. Therefore no one,
&c. Q.E.D.

PROP. XXVI. Whatsoever we endeavour in obedience to reason is
nothing further than to understand; neither does the mind, in so
far as it makes use of reason, judge anything to be useful to it,
save such things as are conducive to understanding.

Proof.--The effort for self--preservation is nothing else but
the essence of the thing in question (III. vii.), which, in so
far as it exists such as it is, is conceived to have force for
continuing in existence (III. vi.) and doing such things as
necessarily follow from its given nature (see the Def. of
Appetite, III. ix. note). But the essence of reason is nought
else but our mind, in so far as it clearly and distinctly
understands (see the definition in II. xl. note. ii.); therefore
(II. xl.) whatsoever we endeavour in obedience to reason is
nothing else but to understand. Again, since this effort of the
mind wherewith the mind endeavours, in so far as it reasons, to
preserve its own being is nothing else but understanding; this
effort at understanding is (IV. xxii. Coroll.) the first and
single basis of virtue, nor shall we endeavour to understand
things for the sake of any ulterior object (IV. xxv.); on the
other hand, the mind, in so far as it reasons, will not be able
to conceive any good for itself, save such things as are
conducive to understanding.

PROP. XXVII. We know nothing to be certainly good or evil, save
such things as really conduce to understanding, or such as are
ab