The Complete Aristotle (eng.)

evil thing (for because of
their instability they unite in bad pursuits, and besides they
become evil by becoming like each other), while the friendship of
good men is good, being augmented by their companionship; and they
are thought to become better too by their activities and by
improving each other; for from each other they take the mould of
the characteristics they approve-whence the saying ‘noble deeds
from noble men’.-So much, then, for friendship; our next task must
be to discuss pleasure.

Nicomachean Ethics, Book X
    Translated by W. D. Ross
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    After these matters we ought perhaps next to discuss pleasure.
For it is thought to be most intimately connected with our human
nature, which is the reason why in educating the young we steer
them by the rudders of pleasure and pain; it is thought, too, that
to enjoy the things we ought and to hate the things we ought has
the greatest bearing on virtue of character. For these things
extend right through life, with a weight and power of their own in
respect both to virtue and to the happy life, since men choose what
is pleasant and avoid what is painful; and such things, it will be
thought, we should least of all omit to discuss, especially since
they admit of much dispute. For some say pleasure is the good,
while others, on the contrary, say it is thoroughly bad-some no
doubt being persuaded that the facts are so, and others thinking it
has a better effect on our life to exhibit pleasure as a bad thing
even if it is not; for most people (they think) incline towards it
and are the slaves of their pleasures, for which reason they ought
to lead them in the opposite direction, since thus they will reach
the middle state. But surely this is not correct. For arguments
about matters concerned with feelings and actions are less reliable
than facts: and so when they clash with the facts of perception
they are despised, and discredit the truth as well; if a man who
runs down pleasure is once seen to be alming at it, his inclining
towards it is thought to imply that it is all worthy of being aimed
at; for most people are not good at drawing distinctions. True
arguments seem, then, most useful, not only with a view to
knowledge, but with a view to life also; for since they harmonize
with the facts they are believed, and so they stimulate those who
understand them to live according to them.-Enough of such
questions; let us proceed to review the opinions that have been
expressed about pleasure.
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    Eudoxus thought pleasure was the good because he saw all things,
both rational and irrational, aiming at it, and because in all
things that which is the object of choice is what is excellent, and
that which is most the object of choice the greatest good; thus the
fact that all things moved towards the same object indicated that
this was for all things the chief good (for each thing, he argued,
finds its own good, as it finds its own nourishment); and that
which is good for all things and at which all aim was the good. His
arguments were credited more because of the excellence of his
character than for their own sake; he was thought to be remarkably
self-controlled, and therefore it was thought that he was not
saying what he did say as a friend of pleasure, but that the facts
really were so. He believed that the same conclusion followed no
less plainly from a study of the contrary of pleasure; pain was in
itself an object of aversion to all things, and therefore its
contrary must be similarly an object of choice. And again that is
most an object of choice which we choose not because or for the
sake of something else, and pleasure is admittedly of this nature;
for no one asks to what end he is pleased, thus implying that
pleasure is in itself an object of choice. Further, he argued that
pleasure when added to any good, e.g. to just or temperate action,
makes it more worthy of choice, and that it is only by itself that
the good can be increased.
    This argument seems to show it to be one of the goods, and no
more a good than any other; for every good is more worthy of choice
along with another good than taken alone. And so it is by an
argument of this kind that Plato proves the good not to be
pleasure; he argues that the pleasant life is more desirable with
wisdom than without, and that if the mixture is better, pleasure is
not the good; for the good cannot become more desirable by the
addition of anything to it. Now it