Are you saying you’re certain that that’s the probability distribution?
Whenever anyone says that some statement’s truth is probable, whether they give a numerical percentage or not, they are making a definite knowledge claim: that the statement’s truth is probable. Whether the probability is measured by a percentage, or relative to some other statement(s), the person is claiming that that is the probability’s measure.
In other words, the person claiming a probability is claiming a certainty about the probability, itself.
If such a person claims that his estimate of the probability is based on another probability, then the question becomes, “What is the prior probability based on?” The failure to provide at least one certainty at the foundation of one’s estimate of such probabilities leads to an infinite regress, or to a baseless circle–neither of which is rationally tenable.
Thus, for any claim of probability to be based in reason, it must be founded on at least one absolute certainty. And so a claim about the probability of any statement’s truth implies at least one claim to certain knowledge.
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Related Posts:
Objectivism vs. Intrinsicism vs. Subjectivism: A Short Summary
The Arbitrary (from The Objectivism Seminar)
Objectivism In Depth 
Great argument. Another relevant point here is that, in probability theory, probability is defined in such a way that the sum of the prior probabilities of the hypotheses under consideration must be equal to 1.
This is pretty intuitive, if you think about it. If I am flipping a coin and estimating the probability that the coin will land on heads, I have to know that the probability that the coin will land on heads plus the probability that the coin will land on tails (plus some tiny probability that it will land on its side, if you like) is equal to 1. If I don’t make that assumption, then I’m leaving open the possibility that something completely different might happen with an unknown prior probability, which destroys the foundation for my estimate.
Again, this is a well established definition that is the foundation for the entire field of probability theory. So yeah, probability depends on certainty.
We need a mid-way between a “certainty, a probability” and a “possibility” (where no such certainty is assummed) and yet it is still rational. Edward de Bono invented a word ‘po’ for that. Po allows – what appears to be irrational into play, but something that is in fact quite rational when the mind is viewed as a self organizing patterning system. Let’s look at examples:
1. Simple use of “PO” – po Trump will get impeached within the year. There is No certainty, no probability being ascertained here. One is merely making statement – not even a hypothesis. It would fall foul of logic until I use the word Po to stabilize this pattern.
2. Practical of use “Po” in creative thinking. PO : a fish will fly into space this year!
The above is obviously absurd but ‘deliberately’ so . Po stabilizes the pattern allowing my mind to self organize into a ‘reasonable movement’- such as :
If an astronaut or any space mission (e.g. even if with robots) takes fish to space then indeed it can be said a fish flew into space (this year) > the above leads to the ideas of populating the moon or other planet with earth animals and seeing how they evolve over time.