The current literature in methodology and epistemology is almost exclusively devoted to one topic: positive evidence, or favourable evidence, or empirical support, or harmony between theory and experience, confirmation, etc. etc. The problem concerning evidence which engages the current literature most is, how does evidence back theory? The present chapter is devoted to the question, what is the good of such backing? The literature views evidence as the basis of rational belief in theories and as the justification of their practical application. Now consider criticism rather than justification to be the key role of rational activity. You will then consider negative evidence as important and tend to view positive evidence plainly as failure, as the undesired outcome of attempts at criticism or at refutation. Since this is not the whole story, the identification of rationality with criticism, as advocated by Popper in his The Open Society and its Enemies, must be rejected. Once we view only internal criticism as rational, once we view rationality as goal-directed and rational criticism only with respect to given ends, then the picture changes. The end of pure science is a theory which is both true and comprehensive or encompassing explanation. Consequently, though negative evidence of a comprehensive explanation is more important than positive evidence, positive evidence is important too — as one which renders theory more comprehensive, or more explanatory.