When Newton’s friends and biographers tried to clarify his views on black pudding and rabbit meat, they weren’t afraid that he’d be thought a closet Jew; they were concerned that he’d be taken for something called a Pythagorean. In the sixth century B.C., Pythagoras of Samos—he of the theorem relating the hypotenuse and the perpendicular sides of a right triangle—founded a community of mystical mathematicians who, it was said, observed a general prohibition against eating animals, “as having a right to live in common with mankind.” Interest in the Pythagorean ban was renewed in the third and fourth centuries A.D. by pagan Neoplatonist philosophers seeking purification of the soul in advance of the afterlife, and it persisted until at least the early nineteenth century. In the sixteenth and seventeenth centuries, the resonance of the term “Pythagorean” was more dietary than mathematical. One explanation of Pythagoreans’ vegetarianism was their adherence to a doctrine known as metempsychosis, or the transmigration of souls. If your soul, after death, could pass into the body of another animal species, vegetarianism was the only sure way to avoid cannibalism.
The History of Vegetarianism.
by STEVEN SHAPIN