 This argument seems to conflate the notions of substratum as pure logical support with that as minute parts. If he means minute parts, then, though it is true that we do not and, in his view, cannot know in detail what they are, we have a theory, which he endorses, that they are probably minute parts as conceived by the atomists, which means they have primary qualities similar in kind though not in scale to those possessed by macroscopic objects. This gives a coherent, though speculative, conception of material substance. Of spiritual substance, we have no similar hypothesis. If, on the other hand, he means pure logical substratum, there is nothing to know, for there is no more to it, in either the material or spiritual case, than its role as substratum. He seems to conflate the ignorance we have of minute parts with the logical emptiness of the idea of pure substratum.