 Qualities are entities of a different type: causal capacities and, for the most part, relations between two bodies (OFQ [1991: 13, 23–30]). The terms “quality” and “power” are treated as synonyms. The quality-making relation is not fully explicit. A relation of congruence, or fit, between the MAs of the two bodies is often mentioned; spatial temporal co-incidence is implicit; connections with other things in the environs are suggested (O’Toole 1974; Kaufman 2006: 166–173). The relational theory is motivated by arguments of two sorts (OFQ [1991: 23–30]). One is illustrated by the legend that the first lock was forged without a key; Boyle argues that when a piece of metal of a certain size and shape was subsequently made, the lock acquired the power to be worked; so the power to be worked and the power to work are relations between the key and the lock; but in the bodies themselves there is nothing distinct from their sizes, figures, motions and those of their parts. The second is an argument from parsimony. If every new quality a body acquired added something to the body itself, it would contain a vast number distinct entities that serve no physical purpose.