 There would be no problem with predicting progress if Leonardo thinks the moral progress lies in the moral preferability of the vegetarian belief rather than the truth of the matter. One might admire vegetarianism without accepting the correctness of vegetarianism. But Leonardo is endorsing the correctness of the belief. This sentence embodies a Moorean absurdity. It is like saying ‘Leonardo took twenty five years to complete The Virgin on the Rocks but I will first believe so tomorrow’. (This absurdity will prompt some to object that I have uncharitably interpreted Leonardo; he must have intended to make an exception for himself and only be referring to men of his kind.)