"The progress of experience, therefore, has dissipated the doubt which must have rested upon the universality of the law of causation, while there were phenomena which seemed to be _sui generis_; not subject to the same laws with any other class of phenomena, and not as yet ascertained to have peculiar laws of their own. This great generalisation, however, might reasonably have been, as it in fact was by all great thinkers, acted upon as a probability of the highest order, before there were sufficient grounds for receiving it as a certainty. For, whatever has been found true in innumerable instances, and never found to be false after due examination in any, we are safe in acting upon as universal provisionally, until an undoubted exception appears; provided the nature of the case be such that a real exception could scarcely have escaped our notice. When every phenomenon that we ever knew sufficiently well to be able to answer the question, had a cause on which it was invariably consequent, it was more rational to suppose that our inability to assign the causes of other phenomena arose from our ignorance, than that there were phenomena which were uncaused, and which happened accidentally to be exactly those which we had hitherto had no sufficient opportunity of studying."--Vol. II. p. 108. _Hypotheses._--Mr Mill's observations on the use of hypotheses in scientific investigation, except that they are characterized by his peculiar distinctness and accuracy of thought, do not differ from the views generally entertained by writers on the subject. We are induced to refer to the topic, to point out what seems to us a harsh measure dealt out to the undulatory theory of light--harsh when compared with the reception given to a theory of Laplace, having for its object to account for the origin of the planetary system.