We have already seen in Chapter 2, “The Yoga of Analysis,” an introduction to the practice of distinguishing matter and spirit. There, the specific issue is the difference between the physical (and mental) body, and the atemporal self. This is a crucial understanding that draws on an ancient Indian philosophical tradition called Sāṅkhya. Indeed, the title of Chapter 2, sāṅkhya-yoga, points to this tradition. Here in Chapter 13 Krishna elaborates on aspects of Sāṅkhya philosophy not mentioned in Chapter 2, and there will be further aspects of this philosophical tradition explained in Chapters 14, 17, and part of Chapter 18.