The mind has its substratum in the Self which is self-evident, i.e. its existence and self-luminosity are obvious.

Ramana Maharishi:  There is the world perceived, the perception is only apparent; it requires location for existence and light. Such existence and light are simultaneous with the rise of mind. So the physical existence and illumination are part of mental existence and illumination. The latter is not absolute, for the mind rises and sinks. The mind has its substratum in the Self which is self-evident, i.e. its existence and self-luminosity are obvious. That is absolute being, continuous in sleep, waking and dream states also.  The world consists of variety, which is the function of the mind. The mind shines by reflected light - i.e. light reflected from the self. Just as the pictures in a cinema show are seen only in diffused, i.e. artificial light, but not in a strong glare or in thick darkness, so also the world pictures are perceptible only in diffused, i.e. reflected light of the Self through the darkness of avidya (ignorance). The world cannot be seen either in pure ignorance as in sleep, or in pure light as in Self-Realisation. Avidya is the Cause of variety.