“Onus probandi incumbit ei qui dicit, non ei qui negat.”
Better known as the Burden Of Proof.
Onus only applies to speech acts. Actually, it only applies to one speech act: assertion. Onus is then about language and disputes. The above actually translates: The burden is on the one declaring, not the one denying.
Onus is not about obligations one has to themselves about what they believe. It only applies to conversations, when a person asserts something is true. However, even if a person asserts, onus doesn’t necessarily apply. If I assert, “The Cowboys are number one!” in a room full of Cowboy fans, no one is going to ask me to justify why I think that’s true. There is no dispute, no disagreement, no denial. The Principle Of Cooperation is the predicate of onus.
Importantly, note that disclosure is a type of speech act that many confuse for asserting, and then think onus exists — “I believe that is true” isn’t an assertion but disclosure about how one is disposed, not asserting what is true of some state of affairs.
Yes, of course we ought to justify all our beliefs, but this ethical obligation is not a conversational obligation, and onus applies to the latter, not the former. In fact, all dispositions including denial require warrant. Another reason this distinction matters. Onus and warrant are very different things.
The difference that makes a difference is that I may have warrant to believe P, but I may not have any ability to assert P; I also may simply disclose my feelings about P; I may not have justification for P but may not have onus when asserting P; in fact, I may have warrant and assert P, yet onus is on the one who doubts P.
“I exist” is an entitlement (and if you don’t like that one, pick any entitlement); I can therefore assert P but there is, by definition, no way to justify asserting P.
“I believe that P” obliges me to have warrant for the belief, but this is not an assertion about P but what I believe about P, and so, bears no onus.
Third, the Dallas Cowboys are number one; everyone’s a fan.
Finally, to spark further thought on the subject, one should ask who has onus when “I exist”, or similar brute facts, are asserted. Is it actually the one adding what is quite obviously the most obvious thing to assert, or the one who may in a fit of solipsism deny the assertion, perhaps for no reason at all? If onus is about a principle of cooperation in conversation, one may have a guide in answering, and an exception to otherwise hard and fast, but unthoughtful rules. What impact has this on Creationists, Flat Earthers, Anti-vaxers, and so on?
“There aren’t any Gods!” and “God exists!” then both bear onus, but not anyone denying either is true; however, anyone with a disposition about either should have warrant for it.
Onus is a principle that applies when we reach a rough spot in the road if an otherwise good conversation; it helps everyone see from another’s vantage point as best they can, knowing at the same facts, as long as they are unambiguously shared.
(See Grice, Onus, Speech Acts, and Implicature)