_Rule of Concomitant Variations_: "When a variation in a given antecedent is accompanied by a variation of a given consequent, they are in some manner related as cause and effect." Atwater says, of the above rules, that "whenever either of these criteria is found, free from conflicting evidence, and especially when several of them concur, the evidence is clear that the cases observed are fair representatives of the whole class, and warrant a valid universal inductive conclusion." We now come to what is known as Hypothesis or Theory, which is an assumed general principle--a conjecture or supposition founded upon observed and tested facts. Some authorities use the term "theory" in the sense of "a verified hypothesis," but the two terms are employed loosely and the usage varies with different authorities. What is known as "the probability of a hypothesis" is the proportion of the number of facts it will explain. The greater the number of facts it will explain, the greater is its "probability." A Hypothesis is said to be "verified" when it will account for all the facts which are properly to be referred to it. Some very critical authorities hold that verification should also depend upon there being no other possible hypotheses which will account for the facts, but this is generally considered an extreme position. A Hypothesis is the result of a peculiar mental process which seems to act in the direction of making a sudden anticipatory leap toward a theory, after the mind has been saturated with a great body of particular facts. Some have spoken of the process as almost _intuitive_ and, indeed, the testimony of many discoverers of great natural laws would lead us to believe that the Subconscious region of the mind is most active in making what La Place has called "the great guess" of discovery of principle. As Brooks says: "The forming of hypotheses requires a suggestive mind, a lively fancy, a philosophic imagination, that catches a glimpse of the idea through the form, or sees the law standing behind the fact."