Plato, Timaeus

Further, as concerns the motions, the best motion of a body is that caused by itself in itself; for this is most nearly akin to the motion of intelligence and the motion of the Universe. Motion due to the agency of another is less good; and the least good motion is that which is imparted to a body lying in a state of rest and which moves it piecemeal and by means of others. Wherefore the motion that is best for purgings and renovations of the body consists in gymnastic exercises; and second-best is the motion provided by swaying vehicles,¹ such as boats or any conveyances that produce no fatigue; while the third kind of motion, although useful for one who is absolutely driven to it, [89b] is by no means acceptable, under any other conditions, to a man of sense, it being the medical kind of purging by means of drugs. For no diseases which do not involve great danger ought to be irritated by drugging. For in its structure every disease resembles in some sort the nature of the living creature. For, in truth, the constitution of these creatures has prescribed periods of life for the species as a whole, and each individual creature likewise has a naturally predestined term of life, [89c] apart from the accidents due to necessity. For from the very beginning the triangles of each creature are constructed with a capacity for lasting until a certain time, beyond which no one could ever continue to live. With respect to the structure of diseases also the same rule holds good: whenever anyone does violence thereto by drugging, in despite of the predestined period of time, diseases many and grave, in place of few and slight, are wont to occur. Wherefore one ought to control all such diseases, so far as [89d] one has the time to spare, by means of dieting rather than irritate a fractious evil by drugging.

Concerning both the composite living creature and the bodily part of it, how a man should both guide and be guided by himself so as to live a most rational life, let our statement stand thus. But first and with special care we must make ready the part which is to be the guide to the best of our power, so that it may be as fair and good as possible for the work of guidance. Now to expound this subject alone in accurate detail would in itself be [89e] a sufficient task.² But treating it merely as a side-issue, if we follow on the lines of our previous exposition, we may consider the matter and state our conclusions not inaptly in the following terms. We have frequently asserted³ that there are housed within us in three regions three kinds of soul, and that each of these has its own motions; so now likewise we must repeat, as briefly as possible, that the kind which remains in idleness and stays with its own motions; in repose necessarily becomes weakest, whereas the kind which exercises itself becomes strongest;