n the action of one part, it is
hindered from attending to a contrary act of the other part.

Reply Obj. 3: The use of reason requires the due use of the
imagination and of the other sensitive powers, which are exercised
through a bodily organ. Consequently alteration in the body hinders
the use of reason, because it hinders the act of the imagination
and of the other sensitive powers.
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FOURTH ARTICLE [I-II, Q. 33, Art. 4]

Whether Pleasure Perfects Operation?

Objection 1: It would seem that pleasure does not perfect operation.
For every human operation depends on the use of reason. But pleasure
hinders the use of reason, as stated above (A. 3). Therefore
pleasure does not perfect, but weakens human operation.

Obj. 2: Further, nothing perfects itself or its cause. But
pleasure is an operation (Ethic. vii, 12; x, 4), i.e. either in its
essence or in its cause. Therefore pleasure does not perfect
operation.

Obj. 3: Further, if pleasure perfects operation, it does so
either as end, or as form, or as agent. But not as end; because
operation is not sought for the sake of pleasure, but rather the
reverse, as stated above (Q. 4, A. 2): nor as agent, because
rather is it the operation that causes pleasure: nor again as form,
because, according to the Philosopher (Ethic. x, 4), "pleasure does
not perfect operation, as a habit does." Therefore pleasure does not
perfect operation.

_On the contrary,_ The Philosopher says (Ethic. x, 4) that "pleasure
perfects operation."

_I answer that,_ Pleasure perfects operation in two ways. First, as an
end: not indeed according as an end is that on "account of which a
thing is"; but according as every good which is added to a thing and
completes it, can be called its end. And in this sense the Philosopher
says (Ethic. x, 4) that "pleasure perfects operation . . . as some end
added to it": that is to say, inasmuch as to this good, which is
operation, there is added another good, which is pleasure, denoting
the repose of the appetite in a good that is presupposed. Secondly, as
agent; not indeed directly, for the Philosopher says (Ethic. x, 4)
that "pleasure perfects operation, not as a physician makes a man
healthy, but as health does": but it does so indirectly; inasmuch as
the agent, through taking pleasure in his action, is more eagerly
intent on it, and carries it out with greater care. And in this sense
it is said in _Ethic._ x, 5 that "pleasures inc