OF METHODS OF ACQUIRING CLEAR SCIENTIFIC IDEAS, _continued._--OF THE DISCUSSION OF IDEAS. APHORISM XXXIII. _The conception involved in scientific truths have attained the requisite degree of clearness by means of the_ Discussions _respecting ideas which have taken place among discoverers and their followers. Such discussions are very far from being unprofitable to science. They are_ metaphysical, _and must be so: the difference between discoverers and barren reasoners is, that the former employ good, and the latter bad metaphysics._ 1. IT is easily seen that in every part of science, the establishment of a new set of ideas has been accompanied with much of doubt and dissent. And by means of discussions so occasioned, the new conceptions, and the opinions which involve them, have gradually become definite and clear. The authors and asserters of the new opinions, in order to make them defensible, have been compelled to make them consistent: in order to recommend them to others, they have been obliged to make them more entirely intelligible to themselves. And thus the Terms which formed the main points of the controversy, although applied in a loose and vacillating manner at first, have in the end become perfectly definite and exact. The opinions discussed have been, in their main features, the same throughout the debate; but they have at first been dimly, and at last clearly apprehended: like the objects of a landscape, at which we look through a telescope ill adjusted, till, by sliding the tube backwards and {181} forwards, we at last bring it into focus, and perceive every feature of the prospect sharp and bright. 2. We have in the last Book[21\3] fully exemplified this gradual progress of conceptions from obscurity to clearness by means of Discussion. We have seen, too, that this mode of treating the subject has never been successful, except when it has been associated with an appeal to facts as well as to reasonings. A combination of experiment with argument, of observation with demonstration, has always been found requisite in order that men should arrive at those distinct conceptions which give them substantial truths. The arguments used led to the rejection of undefined, ambiguous, self-contradictory notions; but the reference to facts led to the selection, or at least to the retention, of the conceptions which were both true and useful. The two correlative processes, definition and true assertion, the formation of clear ideas and the induction of laws, went on together. [Note 21\3: B. **ii. c. ii. Of the Explication of Conceptions.]