A lot has been said about Locke’s account of substance and substratum. Robert Pasnau has recently argued (in his book Metaphysical Themes 1274-1671) that “the substratum just is the ordinary substance” (160). Pasnau says that Locke’s statements about substance become less puzzling when we put them in “the proper historical context, that of the thin metaphysical substance of the Aristotelian tradition” (167, n.9). The “ordinary substance” and the the “thin metaphysical substance of the Aristotelian tradition” are thus identified. The ‘thin’ substance is introduced (101-2) as the union of form and matter, and is then the thing in which accidents inhere. Thin substance plus accidents is the ‘thick’ substance. The thin substance is not “nothing more than a bare substratum”, but is instead “quite rich in character” (107). And “Indeed, in a very real sense, the thin substance just is the cat or dog or stone” (107).
Pasnau says, indeed, that he hopes “that enough has been said to make it seem puzzling why anyone has ever taken seriously the idea of a bare substratum, the unknowable substance beneath the substance” (167). Indeed he suggests that “modern historians have misinterpreted the seventeenth century, and so arrived at a theory of substance that philosophers never would have dreamed of putting forth as their own idea” (167).
There are, however, other contexts here. Yes, we can look at Locke against the thin substance background, but we can also look at him against the background of some of Henry More’s discussions. For some of More’s discussions in The Immortality of the Soul appear to closely parallel the discussions in Essay 2.23. Thus axiom VIII of book I, chapter 2 of The Immortality of the Soul is “The Subject, or naked Essence or Substance of a thing, is utterly unconceivable to any of our Faculties” (More 1959, 10), paralleling the early sections of Essay 2.23, and chapter 3 involves arguing “That the notion of Spirit is altogether as intelligible as that of Body” (More 1659, 16), paralleling the later sections of Essay 2.23.
Moreover, in support of Axiom VIII, More argues as follows: “For the evidencing of this Truth, there needs nothing more then a silent appeal to a mans owne mind, if he doe not find it so; and that if he take away all Aptitudes, Operations, Properties and Modifications from a Subject, that his conception thereof vanishes into nothing, but into the Idea of a meer Undiversificated Substance; so that one Substance is not then distinguishable from another, but onely from Accidents or Modes, to which properly belongs no subsistence” (More 1659, pp.10-1).
Here the substance is indeed the thing distinguished frpm the accidents. But it appears not to be the thin substance of Pasnau’s discussion. On More’s understanding of the subject or substance, it is too thin, so to speak, to be the ordinary cat or horse, for the substance of the cat and the substance of the horse are not distinguishable. This notion of substance at least approaches that of a ‘bare substratum’. And that suggests, at least, that the idea of such a bare featureless underlying substance is not a mere invention of recent commentators, but something that Locke could have found being discussed in his own time and place.