Lucian of Samosata Anacharsis 26

[26] Consequently a man like that cannot help keeping well and holding out protractedly under exhausting labours; it would be long before he would begin to sweat, and he would rarely be found ill. It is as if you should take firebrands and throw them simultaneously into the wheat itself and into its straw and chaff—for I am going back again to the winnower. The straw, I take it, would blaze up far more quickly, while the wheat would burn slowly, not with a great blaze springing up nor at a single burst, but smouldering gradually, until in course of time it too was totally consumed. Neither illness nor fatigue, then, could easily invade and rack such a body, or readily overmaster it; for it has been well stocked within and very strongly fortified against them without, so as not to admit them, nor yet to receive either sun itself or frost to the detriment of the body. To prevent giving way under hardships, abundant energy that gushes up from within, since it has been made ready long beforehand and stored away for the emergency, fills them at once, watering them with vigour, and makes them unwearying for a very long period, for their great preliminary hardships and fatigues do not squander their strength but increase it; the more you fan its flame, the greater it becomes.