Thus those discussions by which scientific conceptions are rendered ultimately quite distinct and fixed, include both reasonings from Principles and illustrations from Facts. At present we turn our attention more peculiarly to the former part of the process; according to the distinction already drawn, between the Explication of Conceptions and the Colligation of Facts. The Discussions of which we here speak, are the Method (if they may be called a _method_) by which the Explication of Conceptions is carried to the requisite point among philosophers. 3. In the _History_ of the Fundamental Ideas of the Sciences which forms the Prelude to this work, and in the _History of the Inductive Sciences_, I have, in several instances, traced the steps by which, historically speaking, these Ideas have obtained their ultimate and permanent place in the minds of speculative men. I have thus exemplified the reasonings and controversies which constitute such Discussion as we now speak of. I have stated, at considerable length, the {182} various attempts, failures, and advances, by which the ideas which enter into the science of Mechanics were evolved into their present evidence. In like manner we have seen the conception of _refracted rays_ of light, obscure and confused in Seneca, growing clearer in Roger Bacon, more definite in Descartes, perfectly distinct in Newton. The _polarity_ of light, at first contemplated with some perplexity, became very distinct to Malus, Young, and Fresnel; yet the phenomena of _circular polarization_, and still more, the _circular polarization of fluids_, leave us, even at present, some difficulty in fully mastering this conception. The _related polarities_ of electricity and magnetism are not yet fully comprehended, even by our greatest philosophers. One of Mr. Faraday's late papers (the Fourteenth Series of his Researches) is employed in an experimental discussion of this subject, which leads to no satisfactory result. The controversy between MM. Biot and Ampère[22\3], on the nature of the Elementary Forces in electro-dynamic action, is another evidence that the discussion of this subject has not yet reached its termination. With regard to _chemical polarity_, I have already stated that this idea is as yet very far from being brought to an ultimate condition of definiteness; and the subject of Chemical Forces, (for that whole subject must be included in this idea of polarity,) which has already occasioned much perplexity and controversy, may easily occasion much more, before it is settled to the satisfaction of the philosophical world. The ideas of the _classificatory_ sciences also have of late been undergoing much, and very instructive discussion, in the controversies respecting the relations and offices of the natural and artificial methods. And with regard to _physiological_ ideas, it would hardly be too much to say, that the whole history of physiology up to the present time has consisted of the discussion of