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.