DT) the inference is that certainty is illusory. Why presume that any particular "Truth" is merely something we may "THINK" we have found? (DTBecause all synthetic truths contain a trace of doubt, and all axiological truths contain a trace of subjectivity. Apodictic certainty outside of analytic truths is indeed illusory. This of course is not to say that all alleged truths are equally uncertain.
DT) the clear presumption is that claims of knowledge are indications of bias. (DTNo, I'd say that the presumption is just that there is generally too little concern and awareness about bias, rather than too much.
DT) it is NOT productive to begin with the bias that firm convictions are necessarily and inherently biased. (DTWell, what fraction of Americans with firm political/religious/sociological convictions would you estimate are subject to serious bias? As staunch libertarians we surely must agree that it's well north of 95%. Expand the sample beyond contemporary America in both space and time, and the fraction gets well north of 99%. Meanwhile, nearly 100% of these firmly-convicted humans have been convinced that they aren't subject to serious bias. Given what we now know about epistemology -- and yes I do believe there is such a thing as knowledge -- we can't claim to have a firm conviction in these areas without being willing to audit for biases the epistemological processes that lead one to claim one's conviction is firmly warranted.
Here are some examples of excellent Overcoming Bias postings: