Why "12" Labors?
We do not know when the number of labors assigned to Hercules (Heracles/Herakles), by Eurystheus, was fixed at 12. Nor do we know if the list we have of the Labors of Hercules contains all the labors ever included, but those we do consider the 12 canonical Labors of Hercules were carved into stone between 470 and 456 B.C. on the metopes of the Doric Temple of Zeus at Olympia -- a spot said to have previously housed the stables of King Augeas, which was the site of one of Hercules' most impressive labors.
Lost Sources for the Complete List
The order and list of the 12 Labors of Hercules may have been recorded earlier, in an epic poem on Hercules (Heracleia), by Peisandros of Rhodes (c. 600 B.C.). However, since this epic is lost (except for a few fragments mentioning the Cerynitian Hind and the Stymphalian Birds) and the remaining poems from the Archaic period only itemize 6 labors, we must turn to later sources for the complete list.
During the Classical period, the labors were known, but each of our literary sources lists fewer than 12. On the Hephaisteion Temple in Athens (built 450-444 B.C.) [see Topography of Ancient Athens], only nine labors were carved, even though there were spots available for 10. It is conceivable that, at the time, nine was the canonical number. It is also possible that some labors we know of are replacements for earlier labors now lost. It wasn't until library writers in the Roman era (Diodorus Siculus and Apollodorus) that we find a complete written list of the canonical 12.
