timaeus   

the body is heated and also cooled within by the elements which
enter into it, and is again dried up and moistened by external things,
and experiences these and the like affections from both kinds of
motions, the result is that the body if given up to motion when in a
state of quiescence is overmastered and perishes; but if any one, in
imitation of that which we call the foster-mother and nurse of the
universe, will not allow the body ever to be inactive, but is always
producing motions and agitations through its whole extent, which
form the natural defence against other motions both internal and
external, and by moderate exercise reduces to order according to their
affinities the particles and affections which are wandering about
the body, as we have already said when speaking of the universe, he
will not allow enemy placed by the side of enemy to stir up wars and
disorders in the body, but he will place friend by the side of friend,
so as to create health.
Now of all motions that is the best which is produced in a thing
by itself, for it is most akin to the motion of thought and of the
universe; but that motion which is caused by others is not so good,
and worst of all is that which moves the body, when at rest, in
parts only and by some external agency. Wherefore of all modes of
purifying and reuniting the body the best is gymnastic; the next
best is a surging motion, as in sailing or any other mode of
conveyance which is not fatiguing; the third sort of motion may be
of use in a case of extreme necessity, but in any other will be
adopted by no man of sense: I mean the purgative treatment of
physicians; for diseases unless they are very dangerous should not
be irritated by medicines, since every form of disease is in a
manner akin to the living being, whose complex frame has an
appointed term of life. For not the whole race only, but each
individual-barring inevitable accidents-comes into the world having
a fixed span, and the triangles in us are originally framed with power
to last for a certain time, beyond which no man prolong his life.
And this holds also of the constitution of diseases; if any one
regardless of the appointed time tries to subdue them by medicine,
he only aggravates and multiplies them. Wherefore we ought always to
manage them by regimen, as far as a man can spare the time, and not
provoke a disagreeable enemy by medicines.
Enough of the composite animal, and of the body which is a part of
him, and of the manner in which a man may train and be trained by
himself so as to live most according to reason: and we must above
and before all provide that the element which is to train him shall be
the fairest and best adapted to that purpose. A minute discussion of
this subject would be a serious task; but if, as before, I am to
give only an outline, the subject may not unfitly be summed up as
follows.
I have often remarked that there are three kinds of soul located
within us, having each of them motions, and I must now repeat in the
fewest words possible, that one part, if remaining inactive and
ceasing from its natural motion, must necessarily become very weak,
but that which is trained and exercised, very strong. Wherefore we
should take care that the movements of the different parts of the soul
should be in due proportion.
And we should consider that God gave the sovereign part of the human
soul to be the divinity of each one, being that part which, as we say,
dwells at the top of the body, inasmuch as we are a plant not of an
earthly but of a heavenly growth, raises us from earth to our
kindred who are in heaven. And in this we say truly; for the divine
power suspended the head and root of us from that place where the
generation of the soul first began, and thus made the whole body
upright. When a man is always occupied with the cravings of desire and
ambition, and is eagerly striving to satisfy them, all his thoughts
must be mortal, and, as far as it is possible altogether to become
such, he must be mortal every whit, because he has cherished his
mortal part. But he who has been earnest in the love of knowledge