大般涅槃經義記 2.198

2.198

下佛答之，以不重生釋成常義。於中，初先簡凡異聖。「所言鐵者名諸凡夫」，推喻屬凡。何故須推？世間之鐵冷熱不恒，佛前取其少分為喻，懼人取其數冷數熱難佛無常，故今推彼數冷數熱不恒之鐵以同凡夫。凡夫之人雖滅煩惱，滅已復生，故名無常，釋凡同喻。「如來不」下，辯聖異凡，以不重生故得名常。

Below, the Buddha answers him, explaining and establishing the meaning of permanence by means of not being reborn. Therein, first he distinguishes the ordinary from the noble. 'What is called iron refers to ordinary beings' assigns the metaphor to ordinary beings. Why must it be so assigned? Iron in the world is not constantly cold or hot; earlier the Buddha took a partial aspect of it as a metaphor, but feared that people would seize upon its repeated cooling and heating to challenge the Buddha with impermanence; therefore, he now assigns that non-constant iron that repeatedly cools and heats to be identical with ordinary beings. Although ordinary beings extinguish afflictions, after being extinguished they are born again, and hence are called impermanent—this explains how ordinary beings match the metaphor. From 'the Tathāgata does not...' onward, he explains how the noble differs from the ordinary: because of not being reborn, he is called permanent.