A newborn has no conception of himself; no notion of a “self.” He barely understands the world around it. As he grows, the infant begins to grasp the world through object permanence, motor coordination, and sensory feedback. He feels internal emotions like happiness, sadness and hunger and starts to recognize that there are objects and people “out there” that interact with him. At this stage, there is still no clear conception of the internal and the external. Although the infant is not yet aware that there is a “him”, a primitive sense of agency begins to emerge through these interactions.
Then, through what Lacan calls the mirror stage, the child starts to develop an image of himself upon seeing its reflection in a mirror. This doesn't have to be a literal mirror, there are other ways he can see himself “from outside”—through the way his mom talks about him, the compliments he gets from his aunts and the things other kids say to him on the playground. In this way, the child develops an ego, a concept of self, and begins to attach words to this new person he just met: “I’m small,” “I’m cute,” “I like toy cars,” “I like playing with the ball.”
But when he accepts these traits he also removes or at least diminishes others. With every trait he accepts, others are excluded: “I’m small” suggests “I’m not big yet,” “I’m cute” suggests “I’m not ugly,” “I like toy cars” suggests “I don’t like dolls,” “I like playing with the ball” suggests “I don’t like staying inside.” The specific traits that get reinforced or repressed vary from person to person, but the underlying process is universal: with every positive definition, countless others are denied. This narrowing, however, is necessary. You can't really navigate life as a shapeless blob of indefiniteness, you need to conceptualize yourself with some specific traits in order to act and make decisions.
Sartre understood that man is pure freedom, pure indefiniteness. Yet, as we established, non-definition isn't something we cope with very well. So we're at odds, we can be anything, but we also must be something, and with every decision we make we're closing off millions of other somethings we might be. Time presses us toward definition; we need an answer and we need it now.
That’s the purpose of the mirror stage. From that moment onward, and for the rest of one’s life, one uses this concept of self to navigate life. The importance does not lie only in the initial construction of the ego, but in the fact that we are continuously at odds with this definition of ourselves. I would argue that this conflict sits at the heart of many emotional and existential struggles humans face. Psychoanalysis may talk too much about what it means to be a baby, but the point is not so much if children develop in some specific way but to try to establish how human consciousness is structured. In this sense, psychoanalysis is closer to metaphysics than psychology.
Many of the emotional struggles we experience arise from conflicts with this imagined ego. I’m not referring here to extreme conditions such as schizophrenia, where the sense of self and even reality itself fall apart. Rather, in most of us, the ego seems somewhat stable and coherent. We all carry an image of ourselves; we might not be able to put it into words, but it is there nonetheless. Every single decision we make is a testament to that.