'Many of the most splendid and important discoveries in this science were the result of analogical reasonings. It was from this source that Dr. Priestley proved the compound nature of atmospheric air; and it is related that it was in consequence of hints which he had given, when on a visit to Paris, to Lavoisier, founded entirely upon analogical conjectures, that the latter philosopher was induced to commence experiments, with the view of proving the compound nature of water, and of reducing it to its constituent elements. Indeed the whole history of this very important and useful department of human knowledge exhibits very striking and incontestable proofs how much of the art owed its existence to mere hints and conjectures, founded, in many cases, upon very slight resemblances or analogies.*. The chief province of analogy is confined to that of suggestion. Analogies are the great hinters of experiments. They illustrate an argument, but do not establish it. They are probabilities, not proofs. Hence Lord Brougham in one place exclaims:--'I have a dread, at least a suspicion, of all analogies, and never more than when on the slippery heights of an obscure subject; when we are, as it were, _inter apices_ of a metaphysical argument, and feeling, perhaps groping, our way in the dark, or among the clouds. I then regard analogy as a dangerous light, a treacherous _ignii fatuus_.'** A striking instance of the fallacy of analogy is afforded in the experiments of Professor Matteuoci, which seem to prove that though the analogies between electricity and nervous substance are nearly perfect, yet they are two distinct agencies.*** * Blakey's Logic, pp. 97-7. ** Pal. Illus. vol. 2. *** See Zoist No. 20, p. 363. CHAPTER XI. DETECTION OF FALLACIES WE hope to be able to save students from the fate of Diodorus, (a great logician, who died in his school through shame at being, unable to resolve a quibble propounded by Stilno)--not by hardening, but by enlightening them. Though we bring neither mood nor figure wherewith to test the presence of error, we are not without the hope of qualifying the student for its discovery. It has been confessed from the throne of logic that, 'After all, in the practical detection of each individual fallacy, _much must depend on natural and acquired acuteness_: nor can any rules be given, the mere learning of which will enable us to _apply_ them with mechanical certainty and readiness.' Bulwer, in remarking that error is a view of _some_ facts instead of a survey of _all_, indicated the key to logical fallacy. Error lies principally in defective premises. Sophistry in science is referable to incomplete analysis of nature, of systems--to artificial arrangements--to _supposing_ qualities, to _assuming_ principles, to false inductions from imperfect demonstration.