One of the things they don’t tell you about having babies is that you don’t ever have a baby; you have your baby, which is, to you, the ur-baby, the sum of all babies. The moment they laid her damp rosy body on my chest, I knew she would envelop my world. I had worried about that very thing. In Sheila Heti’s novel “Motherhood,” the narrator, a cynical writer contemplating whether to have kids before it’s too late, laments the absence of new parents from their friends’ lives, a phenomenon she calls “that relieved and joyful desertion.” “When a person has a child,” she writes, “they are turned towards their child.” The risk of falling off the world haunted me. When you have a baby, you do turn toward your child — that “relieved and joyful desertion” may eventually affect your friends, but it first affects yourself. What I didn’t understand — couldn’t have, at the time — was that deserting yourself for another person really is a relief. My days began to unfold according to her schedule, that weird rhythm of newborns, and the worries I entertained were better than the ones that came before: more concrete, more vital, less tethered to the claustrophobic confines of my own skull. For this member of a generation famously beset by anxiety, it was a welcome liberation. Being young, or young enough still not to know yourself entirely, and then feeling the foundation of your nascent selfhood shift beneath you — perhaps that’s exactly the sort of momentous change that makes the whole enterprise so daunting. Yet there I’ve given up the game: With the exception of — perhaps — a few immutable characteristics, you are not something you discover one day through trial and error and interior spelunking; you are something that is constantly in the process of becoming, the invention of endless revolutions. You never know who you are, because who you are is always changing.