 Why do some things count as admissible substitution instances but not others? More specifically, what is it in virtue of a thing that makes it “resemble” something else? The Mohists do give many illustrations of relevant criteria, making it evident that there are different kinds; shape, color, and function are common examples. As noted above, we judge the squareness of a piece of wood by the similarity of its shape with a carpenter’s T-square, and say that a horse is white because of its color, not its shape. We also judge that a horse is blind because its eyes are impaired in their function.