A sign cannot function at all without producing a physical effect. All our thoughts of every description are signs. A sign is triadic because it determines an interpretant sign of the same object to which it refers itself. A sign is thus a sign of an object, for an interpretant.

[—]

A sign is something which is Secundan to an Object and determines an Interpretant to be correspondingly Secundan to the same Object. But we can distinguish two Objects; the object as it is represented to be, and the object as it is. We can also distinguish three Interpretants; the Interpretant as it is in itself regardless of the sign, the Interpretant as it is actually caused by the sign, and the Interpretant as the sign represents it to be intended.