the fact that a man raises his arm, clenches his fist, and moves his whole arm violently downwards, is a virtue or excellence which is conceived as proper to the structure of the human body. If, then, a man, moved by anger or hatred, is led to clench his fist or to move his arm, this result takes place (as we showed in Pt. II.), because one and the same action can be associated with various mental images of things; therefore we may be determined to the performance of one and the same action by confused ideas, or by clear and distinct ideas. Hence it is evident that every desire which springs from emotion, wherein the mind is passive, would become useless, if men could be guided by reason. Let us now see why desire which arises from emotion, wherein the mind is passive, is called by us blind.
PROP. LX. Desire arising from a pleasure or pain, that is not attributable to the whole body, but only to one or certain parts thereof, is without utility in respect to a man as a whole.
Proof.—Let it be assumed, for instance, that A, a part of a body, is so strengthened by some external cause, that it prevails over the remaining parts (IV. vi.). This part will not endeavour to do away with its own powers, in order that the other parts of the body may perform its office; for this it would be necessary for it to have a force or power of doing away with its own powers, which (III. vi.) is absurd. The said part, and, consequently, the mind also, will endeavour to preserve its condition. Wherefore desire arising from a pleasure of the kind aforesaid has no utility in reference to a man as a whole. If it be assumed, on the other hand, that the part, A, be checked so that the remaining parts prevail, it may be proved in the same manner that desire arising from pain has no utility in respect to a man as a whole. Q.E.D.