Locke, Martin and Substance

I raise three questions concerning Locke's doctrine of substratum , in the light of his correspondence with Stillingfleet: (1) What was his doctrine?(2) Is it philosophically defensible?(3) Is it consistent with his empiricist epistemology? I defend answers which represent Locke's doctrine as being only partially successful but amenable to improvement in certain ways. C.B. Martin has proposed an alternative interpretation of Locke's position. I examine this and find it to be admirable in many respects but implausible in others.