 ‘The founder’ is Plato. The phrase ‘mounted on a holy pedestal’ comes from Plato, Phaedrus 254b7, where the soul has been likened to a charioteer who sees the Forms of the beautiful and temperance so mounted. Some of the phrasing is no doubt neoplatonist rather than Xenocratean, but the formulation, ‘the idea is a paradigmatic cause’, seems to be, as Proclus says, Xenocrates’ attempt to capture Plato’s intent: see here Plato, Parmenides 132d.