Each man is enclosed within himself. None of us, as Leibniz said of the monad, has windows onto the rest of the world. So how do we communicate with one another? By means of external phenomena called signs.

A system of signs is called a language, which can be made up not just of words but of any signs whatever. Although the term “language” is customarily used to refer to spoken words, in our definition the signs of deaf mutes would also qualify.

Scholars often distinguish between natural and artificial signs. The first arise spontaneously, without reflection, while the second develop slowly and are the result of reflection, meditation, and progress. This distinction doesn't lack foundation. Some signs are established deliberately by human will, while others have an instinctual origin. But it's important to pay close attention to the meaning of the word “natural.” Some signs are natural in the sense that they involve spontaneous behavior that, much later in our development, serves to communicate our thoughts. A child laughs if he's happy, for example, and does so spontaneously. Yet if he sees others laugh or cry, he doesn't consider this a sign of joy or happiness, for experience teaches him this only later.

But some have argued that there are natural signs in the proper sense of the word – that for children laughter and crying function as signs and are taken as such even before experience intervenes. Seeing a smiling person approach, doesn't a child himself smile?