The usual account: the brain is tissue, neurons, chemistry; gather enough cells, make the links complex enough, and consciousness will "emerge" on its own. Test the guess on a simple case. One blind man cannot see. Add a second, a third, a million, arrange them however cleverly — sight will not appear; it was in none of them. It is the same with atoms. A single particle is aware of nothing, and no configuration of unconscious particles will yield awareness. You may as well light the room by a complex arrangement of darkness. Consciousness — caitanya — does not accumulate by quantity, nor follow from complexity. Something is either aware or it is not: awareness is an absolute property, and not material.