The proof by exclusion, even when logical, can serve to clench an argument only when other evidence is very strong, and in this instance Dr. Ulmann scarcely attempts to give other corroboration to his thesis. But even in the cases throughout the rest of his work, where he does attempt to prove his assumptions, his conclusions are rendered useless by failure to understand the nature of the evidence required in dealing with works of art -- that is to say, required by connoisseurship. As this is a new science still begging for admission among recognised sciences, a brief parenthesis dealing with the scope of connoisseurship and its methods is scarcely out of place at this point.