It is impossible that two bodies, which are characteristically behaving actively and passively, when they meet, behave simultaneously actively and passively, or that one does not behave actively, and the second - passively. If the bodies are equal in strength, then they are equally active and passive; if one prevails and is stronger than the other, it affects the passive; its activity grows and begins to be felt, and it itself will be almost or not at all passive.
Galen: Die Werke des Galenos.
