Descartes everywhere, and in particular in the theory of aether; excursus on the statue of Ether in the Public Garden; excursus on Rick Deckard; aether as extended substance; how waves work; how waves work in empty space; aetherial mechanics and dynamics; the 11th edition of the Encyclopedia Britannica; Borges; the nature of space, in Kant and in William Gibson; the irrationality of the square root of 2