A concise but brilliant description of how the individual soul is formed from Paramahansa Yogananda:

Spirit projects the desire to create an individualised expression of itself. The soul becomes manifest and projects the idea of the body in causal form. The idea becomes energy; or the lifetronic astral body. The astral body becomes condensed in the physical body. Through the integrated spinal passageway of these three instrumental media, the soul descends into identification with the material body and gross matter.