What is the Third Mind?
M1 is most easily understood (though not necessarily easily recognized) because verbal language is generally a reflection of M1 concepts and functioning.
M2 is understandable because we can observe relationships between parts and a whole, like currents of a river, and characters in a dream.
So, what is M3?
We can say M3 illumines itself as M2. In other words, M2 is of the nature of illumination. This M2 illumination apparently lowers its luminosity through the process of localizing, thus creating the light<->less-light relationships that we detect as localization and difference as M1.
The understanding of light modifies with each configuration of mind. As M1, light generally refers to reflected light, which may be interpreted as physical. As M2, light refers to intrinsic light prior to reflection–the light which lends some of itself to partial recognition as and by M1. The intrinsic, unadulterated illumination that is M2 is the radiance of M3.
By recognizing its relationship with O, M1 can begin moving toward M2. In this movement, the mind begins to integrate, open, and reflect illumination. As M2, consciousness revels in itself, by itself, as itself. In this reveling, it (M2) knows itself (M2) as its (M3's) own radiance.