Creative Speculation

The quote I provided from Carl Sagan, "Absence of evidence is not evidence of absence." demonstrates that one cannot claim something is true without proof, but just because proof doesn't exist doesn't mean "it" doesn't exist. For something to be true, proof must exist; the same is true for something to be "false," proof must exist. Facts require proof. What you are discussing is probability as rationale. I have no issue with it, but it is still not "fact," it is opinion and speculation.

If someone says they don't (do) "believe" in the Yeti because they don't (do) see the possibility; cool. If, however, they say the Yeti doesn't (does) exist and claim it is "fact" because no evidence exists (proving it doesn't exist), then they are flat out wrong.

"The Ancient Greeks might have determined that the Earth was round from the different positions of the stars from different locations on Earth, or by observing the somewhat round horizon of the surface of the sea. They had evidence."

The might have (and now there is speculation a few did), but most in the Ancient World didn't have evidence because they didn't know how to gather it.