"Moral virtue is ... a mean between two vices, that of excess and that of defect, and ... it is no small task to hit the mean in each case, as it is not, for example, any chance comer, but only the geometer, who can find the center of a given circle."

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Galen , in the third section of his book, "The Use of the Limbs," says correctly that it would be in vain to expect to see living beings formed of the blood of menstruous women and the semen virile, who will not die, will never feel pain, or will move perpetually, or shine like the sun. This dictum of Galen is part of the following more general proposition: Whatever is formed of matter receives the most perfect form possible in that species of matter; in each individual case the defects are in accordance with that individual matter.

— Maimonides