Excellence, then, being of these two kinds, intellectual and moral, intellectual excellence owes its birth and growth mainly to instruction, and so requires time and experience, while moral excellence is the result of habit or custom ( ἔθος ), and has accordingly in our language received a name formed by a slight change from ἔθος . 19
From this it is plain that none of the moral excellences or virtues is implanted in us by nature; for that which is by nature cannot be altered by training. For instance, a stone naturally tends to fall downwards, and you could not train it to rise upwards, though you tried to do so by throwing it up ten thousand times, nor could you train fire to move downwards, nor accustom anything which naturally behaves in one way to behave in any other way.