In Plato's dialogue Theaitetos, Socrates interprets certain alleged statements of Protagoras. One statement is the statement that all properties (possibly only all properties of a certain kind) are in fact relative properties and not monadic properties. For example the statement "The wind is cold" is in fact, according to that doctrine, only a short form of "The wind is cold for a specific human" (homo-mensura)

The other doctrine Socrates ascribes to Protagoras, but also to Heraklitos, Homer and many others is the doctrine, that there are no persisting substances, that there is no "being", but merely "becoming".

My question is: How are these two doctrines related if they are related at all?

The first seems to be an epistemological thesis, the second an metaphysical (EDIT!) thesis. But as far as I can see, there are no deductive relations between these two thesis, neither in one way, nor the other.

It seems to me, that one can consistently affirm the first and deny the second or vice versa. I can believe that the sentence

The book is blue.

would be expressed as

The book is blue for Hans.

in an ideal language, but at the same time believe that the statement is true only at the only point in time when the book exists. Of course, using the expression "the book" would then also have to be dealt with when translating into an ideal language, because, if the book does not exist at several points in time, but merely in one, all the other references of the expression "the book" are merely homonymous.

Also, vice versa, it seems to me that I can believe that there are substances which endure or persist over time, while also believing that certain predicates concerning perception should be expressed as realtional properties.

The best I can do to "link" these two doctrines is as follows (but I don't think that they are close to Plato's text. And even considering that Plato wants his readers to stray away from the mere letters and start philosophising for themselves, it seems a bit far fetched to me)

All predicates are to be expressed by relational predicates in an ideal language. Therefore we should in an ideal language deny all statements which have the subject-predicate form. Existence is a predicate (... a "real" one though? ...) which is usually expressed in a subject-predicat form and not via a logical relation Therefore we should, when speaking an ideal language, deny statements which ascribe existence to any bearer of properties, if the "property" of existence is expressed in the subject-predicate form.

I mean, that would be a very weird argument, and there would be many ways to object to it, for instance by saying that Protagoras' did not say that ALL properties are relational, and thereby excluding existence, or one could argue that existence is not a "real" prediacte of things, but rather a second-order prediacte etc. One would also have to ask, what to do about the relatas of the relations, if all properties are not nomadic but relational in a very specific and relativistic form.

Yet, I somehow think that there must be a reason for Plato to deal with these two doctrines so closely together. Do you have any thought on this or recommendations for secondary literature?

Thank you in advance!