On the lowest step of the scale of Nature, cause and effect are quite homogeneous and quite equivalent.
Here therefore we have perfect comprehension of the causal connection: for instance, the cause of the movement of one ball propelled by impact, is the movement of another, which loses just as much movement as the first one receives. Here causality is in the highest degree intelligible.
What notwithstanding still remains mysterious, is restricted to the possibility of the passage of movement—of a thing incorporeal—from one body to another.
The receptivity of bodies in this mode is so slight, that the effect to be produced has to pass over completely from its cause.