 There has been considerable scholarly debate concerning the details of Locke’s account of the distinction. Among the issues are which qualities Locke assigns to each of the two categories. Locke gives several lists. Another issue is what the criterion is for putting a quality in one list rather than another. Does Locke hold that all the ideas of secondary qualities come to us by one sense while the ideas of primary qualities come to us through two or is Locke not making the distinction in this way? Another issue is whether there are only primary qualities of atoms or whether compounds of atoms also have primary qualities. And while Locke claims our ideas of primary qualities resemble the primary qualities in objects, and the ideas of secondary qualities do not resemble their causes in the object, what does ‘resemble’ mean in this context? Related to this issue is how we are supposed to know about particles that we cannot sense. It seems clear that Locke holds that there are certain analogies between the middle sized macroscopic objects we encounter in the world, e.g. porphyry and manna for example, and the particles that compose these things. Maurice Mandelbaum called this process ‘transdiction’. These analogies allow us to say certain things about the nature of particles and primary and secondary qualities. For example we can infer that atoms are solid and that heat is a greater rate of motion of atoms while cold is a slower motion. But these analogies may not get us very far in grasping the necessary connections between qualities in nature. Yet another issue is whether Locke sees the distinction as reductionistic. If what we mean by reductionistic here is that only the primary qualities are real and these explain the secondary qualities then there does not seem to be a clear answer. Secondary qualities surely are nothing more than certain primary qualities that affect us in certain ways. This seems to be reductionistic. But on Locke’s account of “real ideas” in II.30 both the ideas of primary and secondary qualities count as real. And while Locke holds that our ideas of secondary qualities are caused by primary qualities, in certain important respects the primary qualities do not explain them. Locke holds that we cannot even conceive how the size, figure and motion of particles could cause any sensation in us. So, knowing the size, figure and motion of the particles would be of no use to us in this regard (see IV.3.11–40, N: 544–546).