What is the initial encounter that gives realism its primal force? Of anything we can say 'it is and it is something'. Being meets being one might say but when you turn on that tap aporiai begin to flow. Sheltering as they did in the pleroma the pre-socratic and vedic sages could utter:
Om. That (Brahman) is infinite and this (universe) in infinite. The infinite proceeds from the infinite. (Then) taking the infininitude of the infinite (universe), it remains as the infinite (Brahman) alone.(from Brh.Up.V.i.1)
“Nor is it divided, since it is all alike;/ and it is not any more there, which would keep it from holding together,/ nor any worser, but it is all replete with What Is./  Therefore it is all continuous: for What Is approaches What Is (B 7.1)(Parmenides)
Early in both encounters with being there emerged the puzzle: how could being emerge from non-being, how could something which was not cause something which is. The satkaryavadathesis emerged as an early answer within the vedic tradition and was very influential. My note on this theory fragmentary though it is I will post separately.
At this point someone will say that there is no real connection between the pre-socratics and the vedic sages other than that they reside within the ambit of my mind. That for the anti-realist would be perfectly real enough within the meaning of the word. My view is that neither of these groups comes from Mars and that the ingenuity of their individual responses can be analogous. However I would reject outright assimilation. Nobody then knew that they were realists.
But since there is a furthest limit, it is perfected/ from every side, like the bulk of a well-rounded globe,/ from the middle equal every way: for that it be neither any greater/  nor any smaller in this place or in that is necessary;/ for neither is there non-being, which would stop it reaching/ to its like, nor is What Is such that it might be more than What Is/ here and less there. Since it is all inviolate,/ for it is equal to itself from every side, it extends uniformly in limits.
If they weren't realists can they be said to be monists? A commonplace interpretation has been that they both ignore diversity and cling to the nostrum which declares 'all is one'. Not true but there is the difficulty of accounting for the 'many' where there is such a powerful intuition of the 'one'.