the upanishads are lumped with the vedas though they were in production much later, 800 BCE – 400 BCE. those are recorded conversation between the priest and warrior cast discussing the vedas, the meaning of the ritual sacrifice and who is this “one” the vedas talk about.
it is clear that those priests discussing the vedic sacrifice it is not clear what it is all about. rather than tossing the sacrifice aside, this ritual is becoming an internal process. this is the big dawn of yoga. this concept of heat, what is called tapas, is now an internal process. the individual is somewhat burning themselves from the inside, so rather than doing large animal sacrifice to uphold the universe and feed the late ancestors, heat is used to transform the individual. the concept of tapas is key in yoga and also in ayurveda as it is critical to the healing process and heat must be present to sustain a state of balance.
another big take from the upanishads is the concept of karma and trans migration. the soul is eternal and it is preserved through cycles of existence, just as the universe cycles through the 4 yugas. karma, the idea of action is what the buddha had awakened to, the concept of cause an effect.
the last realization, and this is the big one, is the idea of brhaman, this one reality which all is. brhaman is a neuter word in sanskrit, it does not have gender, and brhaman is this underlying reality which is everything in existence. though it may appear to be that we are discreet people, trees, animals and all around us, they are all in fact brhaman. what is described as namarupa – names and forms, these are the infinite manifestations of brhaman.
initially brhaman is described as a monist metaphysics. in the later upanishads a creator god is added to the mix making brhaman a dual and theistic. these are the two main approached, either this one reality has no quality and can only be described negatively. this is the main work of aadi shankara. and a theist approach to brhaman, what is described as suguna brhaman. brhamana with qualities, a creator god we can interact with.