Some animals, especially domesticated animals, are extremely susceptible to signs. A dog will react to the slightest change in the behavior of his master; he will even distinguish the expressions of a human face or the modulations of a human voice. But it is far cry from these phenomena to an understanding of symbolic and human speech.... Symbols—in the proper sense of this term—cannot be reduced to mere signals. Signals and symbols belong to two different universes of discourse: a signal is a part of the physical world of being; a symbol is a part of the human world of meaning. Signals are "operators"; symbols are "designators."