One of the most important and interesting parts of Horsley's letters was that in which he discussed the old objection raised by Priestley that the Christian doctrine of the Trinity was borrowed from Plato. There is, and Horsley does not deny it, a certain resemblance between the Platonic and the Christian theories. The Platonist asserted three Divine hypostases, the Good Being ([Greek: tagathon]), the word or reason ([Greek: logos] or [Greek: noys]), and the Spirit ([Greek: psyche]) that actuates or influences the whole system of the Universe (_anima mundi_), which had all one common Deity ([Greek: to theion]), and were eternal and necessarily existent.[463] Horsley can see no derogation to Christianity in the resemblance of this theory to that of the Christian Trinity. He thinks that the advocates of the Catholic Faith in modern times have been too apt to take alarm at the charge of Platonism. 'I rejoice,' he says, 'and glory in the opprobrium. I not only confess, but I maintain, not a perfect agreement, but such a similitude as speaks a common origin, and affords an argument in confirmation of the Catholic doctrine for its conformity to the most ancient and universal traditions.'[464] For was this idea of a Triad peculiar to Plato? or did it originate with him? 'The Platonists,' says Horsley, 'pretended to be no more than expositors of a more ancient doctrine which is traced from Plato to Parmenides; from Parmenides to his master of the Pythagorean sect; from the Pythagoreans to Orpheus, the earliest of Grecian mystagogues; from Orpheus to the secret lore of Egyptian priests in which the foundations of the Orphic theology were laid. Similar notions are found in the Persian and Chaldean theology; even in Roman superstition from their Trojan ancestors. In Phrygia it was introduced by Dardanus, who carried it from Samothrace.' In short, 'the Trinity was a leading principle in all ancient schools of philosophy and religion.'[465]