The first two stems we should mention are Indo-Germanic and are also found in the Greek and Latin words for “to be.”
1. The oldest and authentic stem word is es, Sanskrit asus, life, the living, that which from out of itself and in itself stands and goes and reposes: the self-standing. To this stem belong the Sanskrit verb forms esmi, esi, esti, asmi. To these correspond the Greek εἶμί and εἶναι and the Latin esum and esse. Sunt, sind and sein belong together. It is worth noticing that the ist (ἔστιν, est, <is> …) persists throughout the Indo-Germanic languages from the very start.
2. The other Indo-Germanic root is bhū, bheu. To this belongs the Greek φύω, to emerge, to hold sway, to come to a stand from out of itself and to remain standing. Until now, bhū has been interpreted according to the usual superficial conception of φύσις and φύειν as nature and as “growing.” According to the more original interpretation, which stems from the confrontation with the inception of Greek philosophy, this “growing” proves to be an emerging which in turn is determined by coming to presence and appearing. Recently, the radical φυ- has been connected with φα-, φαίνεσθαι <to show itself>. φύσις would then be that which emerges into the light, φύειν, to illuminate, to shine forth and therefore to appear. (See Zeitschrift für vergleichende Sprachforschung, vol. 59.)14
From this same stem comes the Latin perfect fui, fuo, as well as our German bin, bist, wir “birn,” ihr “birt” (forms that died [55|76] out in the fourteenth century). The imperative bis (bis mein Weib <be my wife> ) has held out longer next to bin and bist, which have survived.
14. F. Specht, “Beiträge zur griechischen Grammatik,” Zeitschrift für vergleichende Sprachforschung 59 (1932): 31–131. For the connections among bhu¯, pha-, and phu-, see 60 –62.