 In later Mohist thought, “hard-white” or “as hard to white” (jian bai) is a technical term for the relation between two things or two features of a thing that are inseparable and “mutually pervasive”—that is, they completely coincide throughout the same spatial  location.[7]  The paradigm of such features is the hardness and whiteness of a completely white stone. Another example is the length and breadth of an object (B4). Features that are “as hard to white” “fill” or “pervade” each other (B15). Thus they are in a sense “two,” but unlike a pair of shoes, one cannot be taken away from the other (B4). The relation of “as hard to white” contrasts with that between two measured lengths, which cannot everywhere overlap without merging to form only a single length, instead of the original two, and with that between any two mutually exclusive features, such as being an ox and being a horse. No single object can be both an ox and a horse, but an object can be both hard and white. 