Here is a similar idea from George Berkeley, 62 years earlier:
...for myself, I find indeed I have a faculty of imagining, or representing to myself, the ideas of those particular things I have perceived, and of variously compounding and dividing them. I can imagine a man with two heads, or the upper parts of a man joined to the body of a horse. I can consider the hand, the eye, the nose, each by itself abstracted or separated from the rest of the body. But then whatever hand or eye I imagine, it must have some particular shape and colour. Likewise the idea of man that I frame to myself must be either of a white, or a black, or a tawny, a straight, or a crooked, a tall, or a low, or a middle-sized man. I cannot by any effort of thought conceive the abstract idea above described. And it is equally impossible for me to form the abstract idea of motion distinct from the body moving, and which is neither swift nor slow, curvilinear nor rectilinear; and the like may be said of all other abstract general ideas whatsoever.
Rightly (in my opinion) Berkeley points out that, while one can imagine a human being, one cannot imagine a human being without particular qualities of height, color, etc. This illustrates two meanings of the word "abstract." One can imagine an abstract human, or even an abstract body part such as a hand or head, separated from all context and having no basis in reality; but remove all physical description and sensory reference points from the abstract idea, and it is no longer an image of anything.
I imagine the same is true with virtue. When we imagine virtue, we imagine examples of virtue. The abstract ideas of kindness or honesty would mean nothing if they were so far abstracted as to be no longer grounded in human relationships.
George Berkeley. A Treatise Concerning the Principles of Human Knowledge. (1710) Introduction, Paragraph 10.