मोणं चरिस्सामि समिच्च धम्मं, सहिए उज्जुकडे णियाणछिण्णे । संथवं जहिज्ज अकामकामे, अण्णायएसी परिव्वए स भिक्खू ॥१॥
Having reflected upon the Dharma, he who resolves "I will practice monkhood," who is endowed with right knowledge and vision, who is straight and free of deceit, who has cut the cord of craving, who has abandoned attachment to relatives, who is free of desire for sense pleasures, and who wanders seeking alms in unknown households — he is a bhikshu.
This opening sutra establishes the inner resolve of a true monk. The word moṇaṃ is rich: it means not merely silence, but the total orientation of a muni — the entire way of being of one who has renounced. The true bhikshu is not one who merely wears robes, but one who has inwardly severed three things: the rope of craving (niyāṇa), the bond of family attachment (saṃthava), and the pull of sense pleasures (akāmakāme).
The simple version: A true monk is someone who has genuinely decided to live without craving, without attachment to family, and without desire for pleasure, wandering wherever alms may come without preferences.