Digital Companion to C. S. Peirce
1866 | Lowell Lectures on The Logic of Science; or Induction and Hypothesis: Lecture IX | W 1:474

By a ground, […] I meant the pure form or abstraction which is the original of the thing and of which the concrete thing is only an incarnation. Reference to such a ground or respect of likeness is implied in every attribution.

1867 | On a New List of Categories | W 2:52-3; CP 1.551

…the conception of a pure abstraction is indispensable, because we cannot comprehend an agreement of two things, except as an agreement in some respect, and this respect is such a pure abstraction as blackness. Such a pure abstraction, reference to which constitutes a quality or general attribute, may be termed a ground.

1897 [c.] | On Signs [R] | CP 2.228

The sign stands for something, its object. It stands for that object, not in all respects, but in reference to a sort of idea, which I have sometimes called the ground of the representamen. “Idea” is here to be understood in a sort of Platonic sense, very familiar in everyday talk; I mean in that sense in which we say that one man catches another man’s idea, in which we say that when a man recalls what he was thinking of at some previous time, he recalls the same idea, and in which when a man continues to think anything, say for a tenth of a second, in so far as the thought continues to agree with itself during that time, that is to have a like content, it is the same idea, and is not at each instant of the interval a new idea.