We hold these truths to be certain for the only evidence we will admit as evidence is that evidence that is evidence for the truths we hold. Certainty is the last refuge of the prejudiced. For we do not know really what we mean even when we say that the earth is round and not flat. It is round because under the pressure of gravity gas coalesced into a solid object. But what is gravity? Newton thought it was the pneumatic body of Christ, which was probably as good a guess as anybody has come up with yet.