 In dialectical context, Hume is here presupposing (i) an explanatory model familiar from Locke (and Spinoza, Aristotle, Boyle, etc.) in which hidden powers (insensible qualities) of an entity are (causally) responsible for the sensible qualities. So, it is completely natural for early modern natural philosophers to say that these sensible qualities express the essence. These hidden powers may be (part of) the essence of the entity or (as Newton suggests about gravity in the third rule of reasoning) universal qualities of an entity. So, Hume’s implied targets accept something akin to (i). That is not insignificant because (i) is the main explanatory template of early modern science.