What had converged between my mother and her sister, I began to realize, was not personality but its tendency—its first derivative, to borrow a mathematical term. In calculus, the first derivative of a point is not its position in space, but its propensity to change its position; not where an object is, but how it moves in space and time. This shared quality, unfathomable to some, and yet self-evident to a four-year-old, was the lasting link between my mother and her twin. Tulu and Bulu were no longer recognizably identical—but they shared the first derivative of identity.
