But as I've been explaining to unenlightened, there is a fundamental inconsistency between probabilities and impossible. Any impossibility created through probability is not a true impossibility, as the principle of plenitude indicates. — Metaphysician Undercover
I’m in agreement with this.
So maybe you can explain to me what the others have not been able to. Why must there be an underlying psychological certainty? Take your example. I've lost my cup. — Metaphysician Undercover
The leading implicit (psychological) certainty in this hypothetical is that “I’ve lost my cup”. Devoid of this certainty, how would doubts as to where it might be begin manifesting?
I’ll do my best to summarize my position. Both certainty and uncertainty (but not doubt) can be linguistically applied to either a) ontology or b) epistemology. Emotive reasons for such statements aside, when it is said, “It is certain that the planet Earth is not flat,” one here affirms, what I’ll term, an ontic certainty: a determinate state of affairs that thereby holds no alternative possibilities. In contrast, when one says that, “I’m certain that planet Earth is not flat,” one here affirms, what I’ll term, a subjective certainty: a determinate state of mind pertaining to an awareness wherein this awareness deems that which its certain of to be the sole credible possibility, notably, regarding what is in fact ontically certain. Conversely, there are statements such as, “the future’s uncertain,” which can specify an ontic uncertainty: the indeterminate state of affairs that is found in the timespan we term the future. And: “I’m uncertain about the future,” which specifies a subjective uncertainty: an indeterminate state of mind wherein multiple credible alternatives compete for what in fact is the ontically certain (or determinate) state of affairs as it pertains to the future.
I’m hoping that the intelligibility of these four sentences here suffices in substantiating the validity of these two distinct categories of certainty and uncertainty: those which are ontic and those which are subjective.
Ontic certainties and uncertainties are not taken by us to be possibilities but, instead, to be factual states of being. These to me are fancy ways of re-expressing the concept of “is”—which isn’t a possibility but, in our cognition, an absolute. “The cup is on the table” doesn’t express a probability but a fact, which, as facts go, are taken by us to be absolute/total/complete actualities (in so far as they are not mere possibility, or mere potential regarding being). Extreme scenario: the stance that there are no ontic certainties is itself cognized, however implicitly, as depicting that which is ontically certain, thereby resulting in contradiction (hence, an error of reasoning). We think in terms of ontic certainties. A tangential: all ontic uncertainties (i.e., indeterminate states of affairs), if they factually hold presence, shall themselves be ontically certain; e.g. that the future is uncertain, if it is factually so, will itself be a(n ontic) certainty. Otherwise, we couldn’t claim that ontic uncertainties / indeterminate states of affairs factually occur/are.
All subjective certainties (including both psychological and epistemic) will hold some ontic certainty as referent. E.g., if one is certain that planet Earth is not flat one will hold this affirmation to adequately depict that which is ontically certain (or, that which is a determinate state of affairs). The clincher is that no known subjective certainty can be infallible in what it upholds to be ontically certain—but this here seems to be an aside.
With this as general background:
All subjective uncertainties (of which doubt is a type) will likewise be about some given state of affairs, about something which in fact is. This given or set of gives one is uncertain or doubtful about, however, shall itself be subordinate to a subjective certainty (which upholds a referenced ontic certainty): namely, that there is a determinate state of affairs (an ontic certainty) to the subject matter one is uncertain or doubtful about. Devoid of our subjective certainty that there is a relevant, underlying ontic certainty to be discovered, states of uncertainty and doubt become meaningless. This last sentence might be the hardest point to convey given your stances so far, but, as an example: if I am uncertainty/doubtful about whether or not the cup is on the table (or anything else), I already hold a certainty that some cognitive possibility that is conceivable adequately depicts that which is ontically certain regarding this matter. It’s just that I can’t figure out which of the multiple cognitive possibilities I’m pondering this one cognitive possibility is (this for as long as I remain uncertainty/doubtful). Devoid of this underlying subjective certainty that some relevant ontic certainty holds presence, uncertainty and doubt would again not be possible—the multiple alternatives that go through my mind would then not be competing for what in fact is (each, instead, then being its own stable reality, even if they are contradictory to each other).
More briefly, one must first be certain that something is in fact the case in order to be uncertain or in doubt about what the case might in fact be.
Going by the aforementioned, the conclusion is that no subjective uncertainty (including that of doubt) is possible in the complete absence of all subjective certainty. (To me related: also, no ontic uncertainty is possible where it’s presence to not be ontically certain—and, thus, and ontic certainty). Hence, the presence of uncertainty is always subordinate to the presence of certainty.
I’ve condensed my views a lot in this post. Won’t be surprised if there happens to be lack of clarity in what I’ve written. But, if so, point out the pertinent areas where I’ve been less than sufficiently clear.