@reasoning logic,
The phrase "what IS-IS" begs the question about the meaning of the terms "being" and "reality" by assuming that the latter has status independent of observers (aka
naive realism). Pragmatists argue that "reality" is a
social construction insofar that it can have no meaning other than what humans agree "to be the case" in particular co-operative contexts of common need constantly open to historical revision. (The argument is expanded in the Rorty clip).
In addition to the claim by pragmatists for a more coherent position than that of naive realism, there are at least three other nails in the coffin of the absolutist "IS" phrase.
1. Arguments by philosophers of language, particularly Wittgenstein, Quine and Sellers, that
words are not fixed in their meaning but are always relative to context. ( A simplistic way to understand this is to think of words like currency banknotes whose "value" depends on transactional context rather than on the digits printed on them).
2. Arguments that the verb "to be" is so endemically ambiguous that the banning of its usage in favor of a neologism called "e-prime" has been attempted.
3. General existential arguments about the impermanent nature of "being" by Heidegger, Sartre and assorted Postmodernists.
All these rebuttals of absolutist statements, like the one above, are aspects of the modern philosophical trend called "anti-foundationalism" which has as a central tenet that we can never establish any
fixed substrate on which to base what we call "knowledge". In short any attempt at "indubility" (as for example Descartes
cogito) is futile.
IMO Godel's Incompleteness Theorem is the formal expression of anti-foundationalism.
