Edmund Husserl, answered from the texts and cited to the page.
The Other is constituted within transcendental consciousness through analogical apperception of the alien body as a Leib like my own — the perceived body of the other resembles my own lived body in its expressive movements, and a pairing (Paarung) associates them so that the other body is apperceived as bearer of an I like my own.1
The result is transcendental intersubjectivity — the structurally already- shared world that any objective experience presupposes. The Cartesian Meditations Fifth Meditation gives the most-Cartesian formulation; the late research manuscripts (Hua XIII-XV) develop a far more substantial generative phenomenology of historical, cultural, and intergenerational intersubjectivity that the Fifth Meditation only hints at.2345
Quite rightly, therefore, we speak of perceiving someone else and then of perceiving the Objective world, perceiving that the other Ego and I are looking at the same world, and so forth — though this perceiving goes on exclusively within the sphere of my ownness.Cartesian Meditations, pp. 159–160
Ultimately we always get back to the radical differentiation of apperceptions into those that, according to their genesis, belong purely to the primordial sphere and those that present themselves with the sense "alter ego" and, upon this sense, have built a new one — thanks to a genesis at a higher level. 51.Cartesian Meditations, p. 146
This overlaying can bring a total or a partial coincidence, which in any particular instance has its degree, the limiting case being that of complete "likeness".Cartesian Meditations, pp. 147–148
On the contrary, the analogically apprehended object and its indicated system of appearances are indeed analogically adapted to the analogous appearance, which has awakened this whole system too.Cartesian Meditations, p. 154
How does the motivation run? What becomes uncovered as involved in the very complicated intentional performance of the appresentation, which does in fact come about? Initial guidance can be furnished by the verbal sense, an Other: an Other Ego. "Alter" signifies alter ego.Cartesian Meditations, pp. 143–144
Sartre's Being and Nothingness (1943) Part Three on the look became the dominant reading of Husserl's intersubjectivity as solipsistic — the Other as alter ego, derivative from the primordial self. Levinas's Totality and Infinity (1961) extended the worry from a different angle: the Other exceeds the alter ego structure altogether. Theunissen's The Other (1965) provides the comparative study. Zahavi's Husserl und die transzendentale Intersubjektivität (1996) shows that the solipsism charge rests on reading only the Fifth Meditation and ignoring the late research manuscripts (Hua XIII-XV); the late material treats intersubjectivity as transcendentally prior to the constituted ego, not derivative from it.