Today the question is more empirical than philosophical. Conceptual distinctions, contra Descartes, are based on perceptual ones, disembodied mind is a fantasy after the fact. According to Maddy, "the ability to perceive a primitive distinction between a figure and its background is inborn in humans and many laboratory animals. The structure of the retina is probably responsible for the presence of this conceptual information in the human perceptual state, as such a connection has been demonstrated in the case of the frog". In other words, figure/background distinction is an old evolutionary adaptation, and for humans nothing comes before it. This somewhat relaxes Kant's contention that initial manifold of sensation is wholly undifferentiated, but as it turns out not by much.
"Beyond this fairly simple level, however, the evidence is that the capacity to acquire perceptual beliefs of the familiar sort is not present at birth. Psychologists talk of a phenomenon called "identity" in perception. A figure is seen with identity if it is immediately seen as similar to some other figures but dissimilar to others (that is, as falling in some categories but not in others), and it is easily recalled, recognized, or named". This identity recognition is what lies behind differentiation of sensations into distinct unities we call objects. Apparently this type of processing is developmentally acquired, i.e. children learn at young age to differentiate into objects rather than into spectral harmonics say, from their interaction with adults. This is confirmed by experiments, described by Hebb, on people blind from birth, who later acquired eyesight:"Investigators (of vision following operation for congenital cataract) are unanimous in reporting that the perception of a square, circle, or triangle, or of sphere or cube, is very poor. To see one of these as a whole object, with distinctive characteristics immediately evident, is not possible for a long period. The most intelligent and best motivated patient has to seek corners painstakingly even to distinguish a triangle from a circle".
Philosophical attempts to extract "the first distinction" were popular among monistic metaphysical system builders of old, such as Plotinus, Fichte and Hegel. Plotinus writes "The principle of all things is the monad or unit; arising from this monad, the undefined dyad or two serves as material substratum to the monad, which is cause; from the monad and undefined dyad spring numbers". In Fichte's system I posits not-I as a condition of the possibility of self-consciousness, etc. In hindsight it is transparent that "the first principle", be it monad or I, on its own is barren, and the proponents help themselves to their a posteriori empirical experience to conjure up "the first distinction" rather than extract it from the principle "intrinsically".