"There is a principle-not so much a rule of evidence as a presupposition involved in the very conception of a rational system of evidence, as contrasted with the old formal and mechanical systems -which forbids receiving anything irrelevant, not logically probative."1 "The two leading principles should be brought into conspicuous relief, (1) that nothing is to be received which is not logically probative of some matter requiring to be proved; and (2) that everything which is thus probative should come in, unless a clear ground of policy or law excludes it."2