" The second of these quaternios is the negative of the first; it is its shadow. By “shadow” I mean the inferior personality, the lowest levels of which are indistinguishable from the instinctuality of an animal. This is a view that can be found at a very early date, in the idea of the προσϕύης ψʋχή, the ‘excrescent soul’32 of Isidorus.33 We also meet it in Origen, who speaks of the animals contained in man.34 Since the shadow, in itself, is unconscious for most people, the snake would correspond to what is totally unconscious and incapable of becoming conscious, but which, as the collective unconscious and as instinct, seems to possess a peculiar wisdom of its own and a knowledge that is often felt to be supernatural. This is the treasure which the snake (or dragon) guards, and also the reason why the snake signifies evil and darkness on the one hand and wisdom on the other. Its unrelatedness, coldness, and dangerousness express the instinctuality that with ruthless cruelty rides roughshod over all moral and any other human wishes and considerations and is therefore just as terrifying and fascinating in its effects as the sudden glance of a poisonous snake. "

― C.G. Jung , Aion (Collected Works 9ii)