5. Again, some persons condemn all that we have here spoken of as the discussion of ideas, terming it _metaphysical_: and in this spirit, one writer[23\3] has spoken of the 'metaphysical period' of each science, as preceding the period of 'positive knowledge.' But as we have seen, that process which is here termed 'metaphysical,'--the analysis of our conceptions and the exposure of their inconsistencies,--(accompanied with the study of facts,)--has always gone on most actively in the most prosperous periods of each science. There is, in Galileo, Kepler, Gassendi, and the other fathers of mechanical philosophy, as much of _metaphysics_ as in their adversaries. The main difference is, that the metaphysics is of a better kind; it is more conformable to metaphysical truth. And the same is the case in other sciences. Nor can it be otherwise. For all truth, before it can be consistent with _facts_, must be consistent with _itself_: and although this rule is of undeniable authority, its application is often far from easy. The perplexities and ambiguities which arise from our having the same idea presented to us under different aspects, are often difficult to disentangle: and no common acuteness and steadiness of thought must be expended on the task. It would be easy to adduce, from the works of all great discoverers, passages more profoundly metaphysical than any which are to be found in the pages of barren _à priori_ reasoners. [Note 23\3: M. Auguste Comte, _Cours de Philosophie Positive_.] 6. As we have said, these metaphysical discussions are not to be put in opposition to the study of facts; but are to be stimulated, nourished and directed by a constant recourse to experiment and observation. The cultivation of ideas is to be conducted as having for its object the connexion of facts; never to be pursued as a mere exercise of the subtilty of the mind, striving to build up a world of its own, and neglecting that which exists about us. For although man may in this way please himself, and admire the creations of his own brain, he can never, by this course, hit upon the {185} real scheme of nature. With his ideas unfolded by education, sharpened by controversy, rectified by metaphysics, he may _understand_ the natural world, but he cannot _invent_ it. At every step, he must try the value of the advances he has made in thought, by applying his thoughts to things. The Explication of Conceptions must be carried on with a perpetual reference to the Colligation of Facts. Having here treated of Education and Discussion as the methods by which the former of these two processes is to be promoted, we have now to explain the methods which science employs in order most successfully to execute the latter. But the Colligation of Facts, as already stated, may offer to us two steps of a very different kind,--the laws of Phenomena, and their Causes. We shall first describe some of the methods employed in obtaining truths of the former of these two kinds.