Stern's perspective in the book is comprised of his major thesis. His first major emphasis is that the subjective experience of the infant is best made sense of through employing senses of the self as unifying elements of experience across the infant's varied sensory modalities. Each stage of the sense of self unifies more and more complex forms of experience. Stern argues therefore that senses of the self extend far back into infancy, as early as the first two months of life and rejects theories that see infants as having an "undifferentiated" psychology at this stage of life.