Bodhicitta can be defined as the realisation of the nature of emptiness within which effortless compassion arises like heat arising from fire. Compassion, or bodhicitta, is inherent in emptiness as an innate quality, and if one has realised emptiness it is simply impossible that compassion would not be there.

The nature of mind is introduced by way of practice by seeing that all appearances arise from the mind. They are nothing other than the expression of the mind. This must be realised: "objective" appearances all originate from the mind.

The nature of the mind is then realised or introduced to be empty, and the nature of that emptiness, as experienced in meditation, is the primordial wisdom of great bliss. The mind is blissful and by nature empty, and that emptiness is recognised as rigpa, pure awareness.

-- Yangthang Rinpoche