answer that,_ One thing may be necessary for another in four ways.
First, as a preamble and preparation to it: thus instruction is
necessary for science. Secondly, as perfecting it: thus the soul is
necessary for the life of the body. Thirdly, as helping it from
without: thus friends are necessary for some undertaking. Fourthly,
as something attendant on it: thus we might say that heat is
necessary for fire. And in this way delight is necessary for
happiness. For it is caused by the appetite being at rest in the good
attained. Wherefore, since happiness is nothing else but the
attainment of the Sovereign Good, it cannot be without concomitant
delight.

Reply Obj. 1: From the very fact that a reward is given to anyone,
the will of him who deserves it is at rest, and in this consists
delight. Consequently, delight is included in the very notion of
reward.

Reply Obj. 2: The very sight of God causes delight. Consequently, he
who sees God cannot need delight.

Reply Obj. 3: Delight that is attendant upon the operation of the
intellect does not hinder it, rather does it perfect it, as stated in
_Ethic._ x, 4: since what we do with delight, we do with greater care
and perseverance. On the other hand, delight which is extraneous to
the operation is a hindrance thereto: sometimes by distracting the
attention because, as already observed, we are more attentive to
those things that delight us; and when we are very attentive to one
thing, we must needs be less attentive to another: sometimes on
account of opposition; thus a sensual delight that is contrary to
reason, hinders the estimate of prudence more than it hinders the
estimate of the speculative intellect.
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SECOND ARTICLE [I-II, Q. 4, Art. 2]

Whether in Happiness Vision Ranks Before Delight?

Objection 1: It would seem that in happiness, delight ranks before
vision. For "delight is the perfection of operation" (Ethic. x, 4).
But perfection ranks before the thing perfected. Therefore delight
ranks before the operation of the intellect, i.e. vision.

Obj. 2: Further, that by reason of which a thing is desirable, is yet
more desirable. But operations are desired on account of the delight
they afford: hence, too, nature has adjusted delight to those
operations which are necessary for the preservation of the individual
and of the species, lest animals should disregard such operations.
Therefore, in happiness, delight ranks before the operation