There are no pure signs

No sign perhaps, can perfectly realize any one of these types [namely, icon, index, symbol]. They are like chemical elements, which the very laws of chemical reaction prohibit us from obtaining in absolute purity, but to the purification of which we can so far approximate as to get tolerably accurate ideas of their nature, and which present themselves habitually in such a degree of purity, that we have no hesitation in saying, This is gold, that silver, and the other copper; or this is iron, that nickel, and the third cobalt; although all are strictly mixtures of the three. —Charles S. Peirce, MS 599, pp 38f (c.1902)