Dan Stern’s theory of infant development joins his objective knowledge as a developmentalist with his clinical work as a psychoanalyst. The former offers new observational data on the infant’s subjective life while the latter extrapolates upon researcher’s observations with analytic insights and inferences. Several central features of his theory challenge longstanding beliefs within ego psychology. For instance, Stern disputes the entire concept of phases of development linked solely to certain clinical issues, including orality, attachment, autonomy, independence, and trust. In his view, these tasks are central to an individual’s entire lifespan, not just infancy. Instead, his stages of development involve acquiring four separate senses of the self: emergent, core, subjective, and verbal. Once attained, each sense of self continues to function fully and completely throughout life, constantly growing and coexisting. Stern also challenges Margaret Mahler’s theory of an autistic phase of development. According to Stern, infants never undergo an autistic-like phase. Rather, they are born with an emergent sense of self that never becomes confused with the other. Stern also refutes Mahler’s symbiotic phase. From age two to six months, he views infants as attempting to combine a core self as a separate, consistent, delimited, physical entity, with a sense of their own agency, affectivity, and continuity in time. Unification events are seen as an achievement acquired through actively organizing the experience of self-being-with-another, as opposed to the result of a passive inability to differentiate self from other. Finally, Stern refutes Mahler’s perspective on separation-individuation. According to him, infant’s age nine to eighteen months are not principally concerned with achieving independence or autonomy. While he agrees that infants at this age may be exploring the concept of individuation, he believes they are equally interested in finding and forming an intersubjective union with another. They want to know that their thoughts and feelings can be felt by and shared with others. The aforementioned challenges to ego psychology form the basis for Dan Stern’s theory of infant development.