Peirce, On a New List of Categories, § 12
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§ 12. This passage from the many to the one is numerical. The conception of a third is that of an object which is so related to two others, that one of these must be related to the other in the same way in which the third is related to that other. Now this coincides with the conception of an interpretant. An other is plainly equivalent to a correlate. The conception of second differs from that of other, in implying the possibility of a third. In the same way, the conception of self implies the possibility of an other. The Ground is the self abstracted from the concreteness which implies the possibility of an other.
Peirce, On a New List of Categories, CP 1.556, CE 2.55