A sign is something which in some measure and in some respect makes its interpretant the sign of that of which it is itself the sign. [—] [A] sign which merely represents itself to itself is nothing else but that thing itself. The two infinite series, the one back toward the object, the other forward toward the interpretant, in this case collapse into an immediate present. The type of a sign is memory, which takes up the deliverance of past memory and delivers a portion of it to future memory.
http://www.commens.org/dictionary/entry/quote-reasons-rules-4