 I seem to have discovered that the true source of our metaphysical difficulties lies in the attempt, a mistaken attempt too frequent in philosophy, to explain the nature of the universal in terms of something other than itself. In fact the relation of the universal to the particular is something sui generis, presupposed in any explanation of anything. The nature of the universal therefore necessarily and perpetually eludes any attempt to explain itself. The recognition of this enable one to elucidate the whole puzzle of the Parmenides of Plato. (SI, 348, see also SI, 361)