What is the warmth mark?

The warmth sign (ūṣma-gata) among the four auxiliary practices refers to the wisdom state of the mind consciousness and the manas; it is a cognition of the nature of wisdom. Through the mind consciousness's contemplative deliberation and practice (vipaśyanā-caryā), the manas has preliminarily accepted the view that the five aggregates (pañca-skandha) and eighteen realms are without self, though it has not yet been confirmed; thus a warm current arises within, and there is no longer resistance to the principle that the five aggregates are without self. The warmth sign and warm current are not material form (rūpa); they are a metaphor, representing the state and feeling of the mind. Warmth is the state just before fire begins to burn—first the warmth sign appears, and then the fire will catch. This is used as a metaphor for the wisdom state of the manas: having 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.