Why there can't be ghosts (or if there are ghosts they're pretty harmless)

Ghosts and percussion.

"There cannot be any sound or voice where there is no movement or percussion of the air. There cannot be any percussion of the air where there is no instrument. There cannot be any instrument without a body. This being so a spirit cannot have either sound or form or force, and if it should assume a body it cannot penetrate or enter where the doors are shut. And if any should say that through air being collected together and compressed a spirit may assume bodies of various shapes, and by such instrument may speak and move with force, my reply to this would be that where there are neither nerves nor bones there cannot be any force exerted in any movement made by imaginary spirits. Shun the precepts of those speculators whose arguments are not confirmed by experience."

Source: Manuscript B 4v (translation by Edward MacCurdy, The Notebooks of Leonardo da Vinci (New York: George Braziller, 1956), Philosophy chapter.