When Plato defined humans as two-legged animals without feathers, I suspect he was only half serious. Or if he was as humorless as some suppose, his antagonist Diogenes the Cynic certainly picked up on the joke, pointing out that the description sounds pretty much like a plucked chicken. The ancient back and forth illustrates a question that has occupied philosophers for many thousands of years: what separates humans from animals? Is it a soul? Rationality? Tool-making? Most accounts, especially most modern accounts, settle on one crucial central difference—language. Although animals can communicate with each other perfectly well, they do so without this amazingly sophisticated faculty we so often take for granted.
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