Discrimination has to do merely with the senses of terms, and only draws a distinction in meaning.
Separation of Thirdness, or Tertial Separation, called discrimination, consists in representing one of the two separands without representing the other. If A can be prescinded from, i.e. supposed without B, then B can, at least, be discriminated from A.
…one component, B, of the Phaneron may be quite unsupposable (at any rate, by any ordinary mind), without supposing the full and perfect presence of a certain other constituent, A; and yet it may be [that] A may be supposed to be present while B is present in widely varying degrees (or, perhaps sometimes is quite absent). In such a case, we not only get by prescission a concept of A unmixed with B, but we are able to recognize B as a constituent different from its constant and inseparable companion, A. For example, being able to imagine visual surfaces of various colors (or perhaps even colorless, as the boundary between media of widely different indices of refraction; but it is not necessary to resort to such phenomena) we come to recognize color as something quite different from visible superficies, notwithstanding the utter impossibility (for most people) of imagining color without superficial extension. I called this mode of analysis discrimination. It is highly important not to lose sight of it.
In a paper published in the Proceedings of the American Academy of Arts and Sciences for 1867 May 14, I defined the three ways in which an idea can be mentally isolated from another. They [are] Dissociation, Prescission, and Discrimination. [—] Discrimination consists in logically supposing the former idea to be absent though the latter is present. Thus, I can discriminate color from space, since I can logically suppose that there is a space that is uncolored.