 Another issue that has been a matter of controversy since the first publication of the Essay is what Locke means by the term ‘substance’. The primary/secondary quality distinction gets us a certain ways in understanding physical objects, but Locke is puzzled about what underlies or supports the primary qualities themselves. He is also puzzled about what material and immaterial substances might have in common that would lead us to apply the same word to both. These kinds of reflections led him to the relative and obscure idea of substance in general. This is an “I know not what” which is the support of qualities which cannot subsist by themselves. We experience properties appearing in regular clumps, but we must infer that there is something that supports or perhaps ‘holds together’ those qualities. For we have no experience of that supporting substance. It is clear that Locke sees no alternative to the claim that there are substances supporting qualities. He does not, for example, have a theory of tropes (tropes are properties that can exist independently of substances) which he might use to dispense with the notion of substance. (In fact, he may be rejecting something like a theory of tropes when he rejects the Aristotelian doctrine of real qualities and insists on the need for substances.) He is thus not at all a skeptic about ‘substance’ in the way that Hume is. But, it is also quite clear that he is regularly insistent about the limitations of our ideas of substances. Bishop Stillingfleet accused Locke of putting substance out of the reasonable part of the world. But Locke is not doing that.