What is a sign? It is anything which in any way represents an object. This statement leaves us the difficulty of saying what “representing” is. Yet it affords help by pointing out that every sign refers to an object.
A sign does not function as a sign unless it is understood as a sign. It is impossible, in the present state of knowledge, to say, at once fully an precisely, and with a satisfactory approach to certitude, what it is to understand a sign. Consciousness is requisite for reasoning; and reasoning is required for the highest grade of understanding of the most perfect signs; but in view of the facts adduced by Von Hartmann and others concerning Unconscious Mind, it does not seem that consciousness can be considered as essential to the understanding of a sign. But what is indispensible is that there should be an interpretation of the sign; that is that the sign should, actually or virtually, bring about a determination of a sign by the same object of which it itself is a sign. This interpreting sign, like every sign, only functions as a sign so far as it again is interpreted, that is, actually or virtually, determines a sign of the same object of which it is itself a sign. Thus there is a virtual endless series of signs when a sign is understood; and a sign never understood can hardly be said to be a sign.
Is it not essential to a sign’s being a sign that its influence should never cease finally to live, as lending strength to a habit, law, or rule which is ready to produce action when occasion may arise, even although the truth of the sign (if it is a subject of truth or falsehood) be forever denied? [—] In this sense, every sign must be followed by an absolutely endless virtual succession of interpretant signs, or else not be in very truth a sign.
In the light of these considerations it is easy to see that the object of a sign, that to which it, virtually at least, professes to be applicable, can itself only be a sign