The sign is that aspect of a symbol perceivable by the senses.

Two different symbols can share a sign (written or spoken etc.) yet still signify in distinctly different ways. However, a common characteristic of two different objects can never be indicated by using a single sign with two different modes of signification. For the sign, of course, is arbitrary. We could just as well choose two different signs instead, and then where would the common significance be?

In everyday language it is common for the same word to have different modes of signification and thus belong to different symbols. By the same token, two words that have different modes of signification are commonly employed in sentences in what is superficially the same manner. Thus the word 'is' figures as the copula, as a sign for identity, and as an expression for existence. 'Exist' figures as an intransitive verb like 'go', and 'identical' as an adjective. We speak of something, but also say something's happening. (In the sentence, 'Green is green' - where the first word is the proper name of a person and the last an adjective - these words do not merely have different meanings: they are different symbols.)

In this way the most fundamental confusions are easily produced (the whole of philosophy is full of them). In order to avoid such errors, we must use an object language that prevents them. It must do so by not using the same sign for different symbols and by not using signs that have different modes of signification in a superficially similar way. A notation, therefore, that observes logical grammar and has logical syntax. (The conceptual notation of Frege and Russell is such an object language, though, it is true, it fails to exclude all mistakes.)

In order to recognize a symbol by its sign we must mind that it be used in a manner that makes sense. The sign only determines a logical form in conjunction with its logical, syntactic use. If a sign is not needed, it has no meaning: That is the point of Occam's maxim. (If everything acts as if it has meaning, then it does.) The meaning of a sign should never play a role in establishing logical syntax; it must be possible to do it by merely describing expressions.