"When it comes to having a central nervous system, and the ability to feel pain, hunger, and thirst, a rat is a pig is a dog is a boy."

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People must have renounced, it seems to me, all natural intelligence to dare to advance that animals are but animated machines.... It appears to me, besides, that such people can never have observed with attention the character of animals, not to have distinguished among them the different voices of need, of suffering, of joy, of pain, of love, of anger, and of all their affections. It would be very strange that they should express so well what they could not feel.

— Voltaire