Berkeley's Critique of Abstract Ideas and Material Substance

In the Introduction to the Principles of Human Knowledge (1710), Berkeley opens with a sustained attack on the Lockean doctrine of general abstract ideas, which he identifies as the chief source of confusion underlying belief in mind-independent matter. Locke had held, in Book III of the Essay, that general terms such as "triangle" or "man" signify abstract general ideas formed by the mind's stripping away all particular, determinate features from individual triangles or individual men, leaving an idea that is neither equilateral, isosceles, nor scalene, yet somehow represents triangularity as such. Berkeley denies that any such abstract general idea can be formed or even coherently conceived: he challenges the reader to actually form an idea of a triangle that is "neither oblique nor rectangle, equilateral, equicrural nor scalenon, but all and none of these at once," and argues that introspection reveals no such idea is ever genuinely present to the mind, only a particular, determinate idea of some specific triangle used to stand for, or represent, triangles in general in a given context of reasoning.