Because all of our knowledge (beginning with the axiom that reality exists) derives from our experience of reality, that knowledge is knowledge of what is. The mere absence of anything cannot generate any signs or evidence of that absence—and therefore cannot be directly experienced. A corollary of this fundamental fact is the burden-of-proof principle: the burden of proof falls on whoever (in the last analysis) asserts a positive claim—that is, a claim affirming the existence of something. If no evidence can be advanced in support of a positive hypothesis, then logically that hypothesis must be rejected. For if—in violation of the burden-of-proof principle—we accept such a hypothesis as "possible," then we have admitted the arbitrary. Consequently, as we saw in the dialogues above, we can no longer logically claim to know the simplest of truths, even on a probabilistic basis. Consider how this principle applies to the following argument.

DOCTOR A: We should prescribe drug X for our cancer patients. Dozens of reputable independent researchers, in carefully monitored, controlled experiments on thousands of subjects, have found it highly effective, with no observed side-effects. And nothing in its composition suggests potential harm.

DOCTOR B: Oh, I'm familiar with the studies. But how do we know there weren't unnoticed flaws in all this research? We can't afford to expose our patients to the risks of the unknown.

DOCTOR A:	We should prescribe drug X for our cancer patients. Dozens of reputable independent researchers, in carefully monitored, controlled experiments on thousands of subjects, have found it highly effective, with no observed side-effects. And nothing in its composition suggests potential harm.
DOCTOR B:	Oh, I'm familiar with the studies. But how do we know there weren't unnoticed flaws in all this research? We can't afford to expose our patients to the risks of the unknown.