I have taken pains to make my distinction of icons, indices, and tokens clear, in order to enunciate this proposition: in a perfect system of logical notation signs of these several kinds must all be employed. Without tokens there would be no generality in the statements, for they are the only general signs; and generality is essential to reasoning. … But tokens alone do not state what is the subject of discourse ; and this can, in fact, not be described in general terms ; it can only be indicated. The actual world cannot be distinguished from a world of imagination by any description. Hence the need of pronoun and indices, and the more complicated the subject the greater the need of them.

On The Algebra of Logic (1885)