3 9: Sippa Suttaṃ
Translated from the Pali by Thanissaro Bhikkhu.
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[III-9.1]I have heard that on one occasion the Blessed One was staying near Sāvatthī at Jeta's Grove, Anāthapiṇḍika's monastery. Now at that time a large number of monks, after the meal, on returning from their alms round, were sitting gathered together at a pavilion when this discussion arose: "Who, friends, knows a craft? Who's studying which craft? Which is the supreme among crafts?"
With regard to that, some said, "The elephant-craft is the supreme craft among crafts." Some said, "The horse-craft is the supreme craft among crafts" ... "The chariot-craft..." ... "Archery..." ... "Swordsmanship..." ... "Signaling ..." ... "Calculating..." ... "Accounting..." ... "Writing..." ... "Literary composition..." ... "Cosmology..." Some said, "Geomancy is the supreme craft among crafts." And this discussion came to no conclusion.
Then the Blessed One, emerging from his seclusion in the late afternoon, went to the pavilion and, on arrival, sat down on a seat laid out. As he was sitting there, he addressed the monks: "For what topic are you sitting together here? And what was the discussion that came to no conclusion?"
"Just now, lord, after the meal, on returning from our alms round, we were sitting gathered together here at the pavilion when this discussion arose: [They repeat what had been said.]"
"It isn't proper, monks, that sons of good families, on having gone forth out of faith from home to the homeless life, should talk on such a topic. When you have gathered you have two duties: either Dhamma-talk or noble silence."
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
Supporting himself without a craft — light, desiring the goal — his faculties controlled, released everywhere; living in no home, unpossessive, free from longing, having slain Māra, going alone : a monk.
 Reading mudda-sippam with the Commentary. The Thai edition has muddha-sippam, which could mean phrenology, but that doesn't fit in with the previous members of the list, all of which deal with military skills.
See also: SN 46.45