Chemistry, in Natural History, in Physiology;--these are allowed by all to be real, to be great, to be striking; may it not be that the steps of progress in these different cases have in them something alike? May it not be that in each advancing movement of such knowledge there is some common principle, some common process? May it not be that discoveries are made by an _Organ_ which has something uniform in its working? If we can shew that this is so, we shall have the _New Organ_, which Bacon aspired to construct, _renovated_ according to our advanced intellectual position and office. It was with the view of opening the way to such an attempt that I undertook that survey of the past progress of physical knowledge, of which I have given the results in the _History of the Sciences_, and the _History of Scientific Ideas_[1\P]; the former containing the history of the sciences, so far as it depends on {v} observed _Facts_; the latter containing the history of those _Ideas_ by which such Facts are bound into Theories. [Note 1\P: Published in two former editions as part of the _Philosophy of the Inductive Sciences_ (b. i--x.).] It can hardly happen that a work which treats of Methods of Scientific Discovery, shall not seem to fail in the positive results which it offers. For an Art of Discovery is not possible. At each step of the investigation are needed Invention, Sagacity, Genius,--elements which no art can give. We may hope in vain, as Bacon hoped, for an Organ which shall enable all men to construct Scientific Truths, as a pair of compasses enables all men to construct exact circles[2\P]. This cannot be. The practical results of the Philosophy of Science must be rather classification and analysis of what has been done, than precept and method for future doing. Yet I think that the methods of discovery which I have to recommend, though gathered from a wider survey of scientific history, both as to subjects and as to time, than (so far as I am aware) has been elsewhere attempted, are quite as definite and practical as any others which have been proposed; with the great additional advantage of being the methods by which all great discoveries in science have really been made. This may be said, for instance, of _the Method of Gradation_ and _the Method of Natural Classification_, spoken of b. iii. c. viii; and in a narrower sense, of _the Method of Curves_, _the Method of_ {vi} _Means_, _the Method of Least Squares_ and _the Method of Residues_, spoken of in chap. vii. of the same Book. Also the Remarks on the _Use of Hypotheses_ and on the _Tests of Hypotheses_ (b. ii. c. v.) point out features which mark the usual course of discovery. [Note 2\P: _Nov. Org._ lib. i. aph. 61.]