of excess, all violent and unpleasant emotion, all mental overstrain, take daily exercise in the open air, cold baths and such like hygienic measures.  For without a proper amount of daily exercise no one can remain healthy; all the processes of life demand exercise for the due performance of their functions, exercise not only of the parts more immediately concerned, but also of the whole body.  For, as Aristotle rightly says, Life is movement; it is its very essence.  Ceaseless and rapid motion goes on in every part of the organism.  The heart, with its complicated double systole and diastole, beats strongly and untiringly; with twenty-eight beats it has to drive the whole of the blood through arteries, veins and capillaries; the lungs pump like a steam-engine, without intermission; the intestines are always in peristaltic action; the glands are all constantly absorbing and secreting; even the brain has a double motion of its own, with every beat of the pulse and every breath we draw.  When people can get no exercise at all, as is the case with the countless numbers who are condemned to a sedentary life, there is a glaring and fatal disproportion between outward inactivity and inner tumult.  For this ceaseless internal motion requires some external counterpart, and the want of it produces effects like those of emotion which we are obliged to suppress.  Even trees must be shaken by the wind, if they are to thrive.  The rule which finds its application here may be most briefly expressed in Latin:  omnis motus, quo celerior, eo magis motus.

How much our happiness depends upon our spirits, and these again upon our state of health, may be seen by comparing the influence which the same external circumstances or events have upon us when we are well and strong with the effects which they have when we are depressed and troubled with ill-health.  It is not what things are objectively and in themselves, but what they are for us, in our way of looking at them, that makes us happy or the reverse.  As Epictetus says, Men are not influenced by things, but by their thoughts about things.  And, in general, nine-tenths of our happiness depends upon health alone.  With health, everything is a source of pleasure; without it, nothing else, whatever it may be, is enjoyable; even the other personal blessings,—­a great mind, a happy temperament—­are degraded and dwarfed for want of it.  So it is really with good reason that, when two people meet, the first thing they do is to inquire after each other’s health, and to express the hope that it is good; for good health is by far the most important element in human happiness.  It follows from all this that the greatest of follies is to sacrifice health for any other kind of happiness, whatever it may be, for gain, advancement, learning or fame, let alone, then, for fleeting sensual pleasures.  Everything else should rather be postponed to it.