There can be no voice where there is no motion or percussion of the air; there can be no percussion of the air where there is no instrument, there can be no instrument without a body; and this being so, a spirit can have neither voice, nor form, nor strength. And if it were to assume a body it could not penetrate nor enter where the passages are closed. And if any one should say that by air, compressed and compacted together, a spirit may take bodies of various forms and by this means speak and move with strength—to him I reply that when there are neither nerves nor bones there can be no force exercised in any kind of movement made by such imaginary spirits.
Beware of the teaching of these speculators, because their reasoning is not confirmed by experience.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.