I. "Whenever we can alter the quantity of the things experimented on, we can apply _a rule for discovering which are causes and which are effects_, as follows: We must vary the quantity of one thing, making it at one time greater and at another time less, and if we observe any other thing which varies just at the same times, it will in all probability be an _effect_." II. "When things vary regularly and frequently, there is _a simple rule, by following which we can judge whether changes are connected together as causes and effects_, as follows: Those things which change in exactly equal times are in all likelihood connected together." III. "It is very difficult to explain how it is that we can ever reason from one thing to a class of things by generalization, _when we cannot be sure that the things resemble each other in the important points_.... Upon what grounds do we argue? We have to get a general law from particular facts. This can only be done by going through all the steps of inductive reasoning. Having made certain observations, we must frame hypotheses as to the circumstances, or laws from which they proceed. Then we must reason deductively; and after verifying the deductions in as many cases as possible, we shall know how far we can trust similar deductions concerning future events.... It is difficult to judge when we may, and when we may not, safely infer from some things to others in this simple way, without making a complete theory of the matter. _The only rule_ that can be given to assist us is that _if things resemble each other in a few properties only, we must observe many instances before inferring that these properties will always be joined together in other cases_." CHAPTER XIV. MAKING AND TESTING HYPOTHESES