  Which qualities exactly would we be able to deduce? We would be able to deduce a substance’s tertiary qualities, that is, its powers to produce certain effects in other  substances.[13] If we knew the real essences of opium, and of hemlock, then just as though we were performing a geometric deduction, or just as a locksmith understands why a given key will open one lock but not another, we could deduce that opium produces sleep, that hemlock causes death, and we would understand why each substance produces its effects.