David Armstrong's theory of universals is sparse: it's "mass", "electric charge", etc. that are true universals, not e.g. "redness".
In pre-modern times this might be vaguely comparable to the classical elements, Earth, Water, Air, Fire - in the sense of: If there are any universals (nominalism is false), at least those must be.
There was also a further simple "grouping" (?) of classical elements into sensible qualities by Aristotle: "hot", "cold", "dry" and "wet".
So we get the universals hotness, coldness, dryness and wetness.
But there is a problem here: How can those be more than predicates (mere expressions that can be true of something)? Dryness is "reducible" to the Earth and Fire universals: Dryness is present in a particular iff Fire and/or Earth predominate in it.
And analogously we can invent a combination like of Air and Earth, or, of Fire and Water, which do not give us "sensible qualities".
Presumably "Earthairiness" or "Firewaterness" is not eligible to be a universal. But what makes it so?
So there is a problem of distinguishing between a predicate, that results from a mere logical (conjunction, disjunction, ...) combination of universals (or a more complex description of how a particular is), and true universals in their own right.
Since the problem of universals stretches from Aristotle to Armstrong, is there a set of "standard solutions" to answer this?
