Ahhh.. now we see.. datum IS the abstract concept. Which, as noted, leads to a fairly obvious 'truth' when defined that way....in fact, 'data', which I carelessly call 'facts'- are most numerous 'entities' in the Universe.

"On a planet going round a star, 327 million light years away, there is a small piece of rock which is 27.39 cm. from a slightly larger rock."

Well, probably, anyway.

We don't need to go that far. Every relationship of every atom here on Earth to every other atom is, I suppose, a 'datum' if someone contemplates it. What being an ...ahem.. advanced life-form is all about is defining and relating to the relevant data, and then composing conceptual statements ABOUT those relationships, thus creating new data....in an endless spiral. Now THAT is what the idea of ∞ is good for!

   So...I am willing to concede that definition of a 'datum' as something that doesn't 'exist' without a mind holding it. The 'relationship' of one atom in the Universe to another is still there, whether it is 'conceptualized' or not. Perhaps you'd wish 'datum' to indicate something conceptually different from 'fact'...*shrug*. Once we have defined these things... then what?

(All these things AS data, mean job security for philosophers...and THAT makes it all relevant!)