The perfect identity between cause and effect, as the causal tendency postulates it, would imply from all evidence their equivalence — that is, the possibility of reversing the phenomenon, of arriving at the antecedent by starting from the consequent. On the other hand, this "reversibility," as we say in physics, does not imply identity. I can exchange a tenfranc gold piece for two five-franc silver pieces, or vice versa, from which it follows that these things have the same value, are equivalent, but not identical.