The thesis of this post is: "Each 'is' claim relies implicitly or explicitly on at least one 'ought' claim."

I will walk through a series of arguments that suggest that this claim is true, and then flesh out the picture towards the end.

(note: I discovered after writing this post that my argument is similar to Cuneo's argument for moral realism; I present it anyway in the hope that it is additionally insightful)

Epistemic virtue

There are epistemic virtues, such as:

Try to have correct beliefs.
When you're not sure about something, see if there's a cheap way to test it.
Learn to distinguish between cases where you (or someone else) is rationalizing, versus when you/they are offering actual reasons for belief.
Notice logical inconsistencies in your beliefs and reflect on them.
Try to make your high-level beliefs accurately summarize low-level facts.

These are all phrased as commands, which are a type of ought claim. Yet, they all assist one following such commands to have more accurate beliefs.

Indeed, it is hard to imagine how someone who does not (explicitly or implicitly) follow rules like these could come to have accurate beliefs. There are many ways to end up in lala land, and guidelines are essential for staying on the path.

So, "is" claims that rely on the speaker of the claim having epistemic virtue to be taken seriously, rely on the "ought" claims of epistemic virtue itself.

Functionalist theory of mind

The functionalist theory of mind is "the doctrine that what makes something a mental state of a particular type does not depend on its internal constitution, but rather on the way it functions, or the role it plays, in the system of which it is a part." For example, according to functionalism, for myself to have a world-representing mind, part of my brain must be performing the function of representing the world.

I will not here argue for the functionalist theory of mind, and instead will assume it to be true.

Consider the following "is" claim: "There is a plate on my desk."

I believe this claim to be true. But why? I see a plate on my desk. But what does that mean?

Phenomenologically, I have the sense that there is a round object on my desk, and that this object is a plate. But it seems that we are now going in a loop.

Here's an attempt at a way out. "My visual system functions to present me with accurate information about the objects around me. I believe it to be functioning well. And I believe my phenomenological sense of there being a plate on my desk to be from my visual system. Therefore, there is a plate on my desk."

Well, this certainly relies on a claim of "function". That's not an "ought" claim about me, but it is similar (and perhaps identical) to an "ought" claim about my visual system: that presenting me with information about objects is what my visual system ought to do.

Things get hairy when examining the second sentence. "I believe it to be functioning well." Why do I believe that?

I can consider evidence like "my visual system, along with my other sensory modalities, presents me with a coherent world that has few anomalies." That's a complex claim, and checking it requires things like checking my memories of how coherent the world my senses present to me is, which is again relying on the parts of my mind to perform their functions.

I can't doubt my mind except by using my mind. And using my mind requires, at least tentatively, accepting claims like "my visual system is there for presenting me with accurate information about the objects around me."

Indeed, even making sense of a claim such as "there is a plate on my desk" requires me to use some intuition-reliant faculty I have of mapping words to concepts; without trust in such a faculty, the claim is meaningless.

I, therefore, cannot make meaningful "is" claims without at the same time using at least some parts of my mind as tools, applying "ought" claims to them.

Social systems

Social systems, such as legal systems, academic disciplines, and religions, contain "ought" claims. Witnesses ought to be allowed to say what they saw. Judges ought to weigh the evidence presented. People ought not to murder each other. Mathematical proofs ought to be checked by peers before being published.

Many such oughts are essential for the system's epistemology. If the norms of mathematics do not include "check proofs for accuracy" and so on, then there is little reason to believe the mathematical discipline's "is" claims such as "Fermat's last theorem is true."

Indeed, it is hard for claims such as "Fermat's last theorem is true" to even be meaningful without oughts. For, there are oughts involved in interpreting mathematical notation, and in resolving verbal references to theorems. Such as, "the true meaning of '+' is integer addition, which can be computed using the following algorithm."

Without mathematical "ought"s, "Fermat's last theorem is true" isn't just a doubtful claim, it's a meaningless one, which is not even wrong.

Language itself can be considered as a social system. When people misuse language (such as by lying), their statements cannot be taken seriously, and sometimes can't even be interpreted as having meaning.

(A possible interpretation of Baudrillard's simulacrum theory is that level 1 is when there are sufficient "ought"s both to interpret claims and to ensure that they are true for the most part; level 2 is when there are sufficient "ought"s to meaningfully interpret claims but not to ensure that they are true; level 3 is when "ought"s are neither sufficient to interpret claims nor to ensure that they are true, but are sufficient for claims to superficially look like meaningful ones; and level 4 is where "ought"s are not even sufficient to ensure that claims superficially look meaningful.)

Nondualist epistemology

One might say to the arguments so far:

"Well, certainly, my own 'is' claims require some entities, each of which may be a past iteration of myself, a part of my mind, or another person, to be following oughts, in order for my claims be meaningful and/or correct. But, perhaps such oughts do not apply to me, myself, here and now."

However, such a self/other separation is untenable.

Suppose I am a mathematical professor, who is considering performing academic fraud, to ensure that false theorems end up in journals. If I corrupt the mathematical process, then I cannot, in the future, rely on the claims of mathematical journals to be true. Additionally, if others are behaving similarly to me, then my own decision to corrupt the process is evidence that others also decide to corrupt the process. Some of these others are in the past; my own decision to corrupt the process is evidence that my own mathematical knowledge is false, as it is evidence that those before me have decided similarly. So, my own mathematical "is" claims rely on myself following mathematical "ought" claims.

(More precisely, both evidential decision theory and functional decision theory have a notion by which present decisions can have past consequences, including past consequences affecting the accuracy of presently-available information)

Indeed, the idea of corrupting the mathematical process would be horrific to most good mathematicians, in a quasi-religious way. These mathematicians' own ability to take their work seriously enough to attain rigor depends on such a quasi-religious respect for the mathematical discipline.

Nondualist epistemology cannot rely on a self/other boundary by which decisions made in the present moment have no effects on the information available in the present moment. Lying to similar agents, thus, undermines both the meaningfulness and the truth of one's own beliefs.

Conclusion

I will summarize the argument thusly:

Each "is" claim may or may not be justified.
An "is" claim is only justified if the system producing the claim is functioning well at the epistemology of this claim.
Specifically, an "is" claim that you make is justified only if some system you are part of is functioning well at the epistemology of that claim. (You are the one making the claim, after all, so the system must include the you who makes the claim)
That system (that you are part of) can only function well at the epistemology of that claim if you have some function in that system and you perform that function satisfactorily. (Functions of wholes depend on functions of parts; even if all you do is listen for a claim and repeat it, that is a function)
Therefore, an "is" claim that you make is justified only if you have some specific function and you expect to perform that function satisfactorily.
If a reasonable agent expects itself to perform some function satisfactorily, then according to that agent, that agent ought to perform that function satisfactorily.
Therefore, if you are a reasonable agent who accepts the argument so far, you believe that your "is" claims are only justified if you have oughts.

The second-to-last point is somewhat subtle. If I use a fork as a tool, then I am applying an "ought" to the fork; I expect it ought to function as an eating utensil. Similar to using another person as a tool (alternatively "employee" or "service worker"), giving them commands and expecting that they ought to follow them. If my own judgments functionally depend on myself performing some function, then I am using myself as a tool (expecting myself to perform that function). To avoid self-inconsistency between myself-the-tool-user and myself-the-tool, I must accept an ought, which is that I ought to satisfactorily perform the tool-function I am expecting myself to perform; if I do not accept that ought, I must drop any judgment whose justification requires me to perform the function generating this ought.

It is possible to make a similar argument about meaningfulness; the key point is that the meaningfulness of a claim depends on the functioning of an interpretive system that this claim is part of. To fail to follow the oughts implied by the meaningfulness of ones' statements is not just to be wrong, but to collapse into incoherence.

Certainly, this argument does not imply that all "ought"s can be derived from "is"es. In particular, an agent may have degrees of freedom in how it performs its functions satisfactorily, or in doing things orthogonal to performing its functions. What the argument suggests instead is that each "is" depends on at least one "ought", which itself may depend on an "is", in a giant web of interdependence.

There are multiple possible interdependent webs (multiple possible mind designs, multiple possible social systems), such that a different web could have instead come in to existence, and our own web may evolve into any one of a number of future possibilities. Though, we can only reason about hypothetical webs from our own actual one.

Furthermore, it is difficult to conceive of what it would mean for the oughts being considered to be "objective"; indeed, an implication of the argument is that objectivity itself depends on oughts, at least some of which must be pre-objective or simultaneous with objectivity.

Related, at least some of those oughts that are necessary as part of the constitution of "is", must themselves be pre-"is" or simultaneous with "is", and thus must not themselves depend on already-constituted "is"es. A possible candidate for such an ought is: "organize!" For the world to produce a map without already containing one, it must organize itself into a self-representing structure, from a position of not already being self-representing. (Of course, here I am referring to the denotation of "organize!", which is a kind of directed motion, rather than to the text "organize!"; the text cannot itself have effective power outside the context of a text-interpretation system)

One can, of course, sacrifice epistemology, choosing to lie and to confuse one's self, in ways that undermine both the truth and meaningfulness of one's own "is" claims.

But, due to the anthropic principle, we (to be a coherent "we" that can reason) are instead at an intermediate point of a process that does not habitually make such decisions, or one which tends to correct them. A process that made such decisions without correcting them would result in rubble, not reason. (And whether our own process results in rubble or reason in the future is, in part, up to us, as we are part of this process)

And so, when we are a we that can reason, we accept at least those oughts that our own reason depends on, while acknowledging the existence of non-reasoning processes that do not.