What is the sign of warmth?

The warmth sign (ūṣma-gata) among the four aids to penetration refers to the wisdom realm of the mind consciousness and the manas; it is a cognition of the nature of wisdom. Through the mind consciousness's deliberation and contemplative practice (vipaśyanā-caryā), the manas has preliminarily accepted the view that the five aggregates (pañcaskandha) and the eighteen realms are without self, though not yet with certainty; thus a stream of warmth has arisen within, no longer resisting the principle that the five aggregates are without self. The warmth sign and the stream of warmth are not material form (rūpa); they are a metaphor, representing the realm and feeling of the mind. "Warmth" is the state just before fire is about to burn: first the warmth sign appears, and then the fire will catch. By analogy, this refers to the wisdom realm of the manas, which has gained a certain cognition of the Buddha-dharma and preliminarily accepted the principle that the five aggregates are without self; only afterward can definite faith and realization arise.