Temet Nosce is the Latin form of the original Greek epithet Gnothi Seauton (meaning Know Thyself), which is advice is carved (some say in gold, others say in stone) above the entrance of a temple built to Apollo, in the Greek city of Delphi. This temple was the attending place of an Oracle, a prophetess who served as a channel that brought the will and opinions of Apollo to the ones who came seek for his advice, concerning personal decisions or warfare and commercial voyages.
Recent discovers by North American geologists indicate that the visions and prophecies may had been the result of neurological disturbances effected by a fume that used to came from the innards of the Earth, up to the surface through a hole in the ground inside the temple.
It's interesting to note that, while the original is carved in gold in the temple of Apollo, it is this Latin version of the phrase that's carved in wood above the entrance of the Oracle's kitchen in The Matrix. A simulacra in a simulacra.