The fact is that on that night I laughed at the axiom Quae sunt aequalia uni tertio sunt aequalia inter se (“Things which are equal to a third thing are equal to each other”), for the portrait resembled M. M. and it also resembled the strumpet, and the latter did not resemble M. M. Murray admitted it, and we spent an hour philosophizing.
Giacomo Casanova, History of My Life, volume 4, chapter 10