This paper argues that “YOU” and “I” (“I” and “THOU”) are fundamental elements of human thought, present as distinct words (or signs) in all human languages. I first developed this thesis in my 1976 article “In defense of YOU and ME” (and before that, introduced it in my 1972 book Semantic Primitives; cf. also my 2021 article “‘Semantic Primitives’, fifty years later”). Since then, it has been confirmed by wide-ranging cross-linguistic investigations conducted in the Natural Semantic Metalanguage (NSM) framework. But neither the truth of this thesis nor its importance have become widely recognised in linguistics or anthropology. Influential scholars in both these fields continue to undermine the notion of the fundamental unity of humankind and to put total emphasis, instead, on the diversity of languages and cultures. As cross-linguistic investigations of the last fifty years show, however, despite the phenomenal diversity of human languages a shared “alphabet of human thoughts” was not just a figment of Leibniz’s imagination but a fitting metaphor for something real and immeasurably important. As the present article aims to show, “YOU” and “I” (“I” and “THOU”) are two twin cornerstones of this reality. To quote the entry on “Psychic unity of humankind” in the Encyclopedia of Anthropology, “Ineluctably, the idea [of a deep psychological unity of humankind] has ethical significance. For attempting to inform humans about what they are and what they have in common is not a neutral act” (Prono 2006). As the present article seeks to demonstrate (and as Martin Buber compellingly affirmed a century ago), “I” and “THOU” are an ineluctable part of who we are: how we think, how we speak and how we relate to others.