II. His Ruling Symbols from The Philosophy of Shelley’s Poetry in Ideas of Good and Evil by William Butler Yeats.

“It may be that his subconscious life seized upon some passing scene, and moulded it into an ancient symbol without help from anything but that great memory; but so good a Platonist as Shelley could hardly have thought of any cave as a symbol, without thinking of Plato’s cave that was the world; and so good a scholar may well have had Porphyry on ‘the Cave of the Nymphs’ in his mind.” [via]