To recognise the identity of phenomena which thus differ only in degree, we must assume a type-species. This will be that _kind_ which has the class-properties in their greatest intensity (and, therefore, most easily studied with all their effects); and we must conceive the other varieties as instances of degeneracy from that type. The divisions of the series must be determined by the principles of _natural_ grouping in general (that is, in effect, by natural affinity); in subordination, however, to the principle of a natural series; that is, in the same group must not be placed things which ought to occupy different points of the general scale. Zoology affords the only _complete_ example of the true principles of rational classification, whether as to the formation of groups or of series. Yet the same principles are applicable to all cases (to art and business as well as science) where the various parts of a wide subject have to be brought into mental co-ordination. BOOK V. FALLACIES. CHAPTER I. FALLACIES IN GENERAL. The habit of reasoning well is the only complete safeguard against reasoning ill, that is, against drawing conclusions with insufficient evidence, a practice which the various contradictory opinions, particularly about the phenomena relating to Man, show to be even now common, and that too among the most enlightened. But, to be able to explain an error is a necessary condition of seeing the truth; for, 'Contrariorum eadem est Scientia.' Consequently, a work on Logic must classify Fallacies, that is, the varieties of Apparent Evidence; for they _can_ be classified, though not in respect of their negative quality of being either not evidence at all, or inconclusive, yet in respect of the positive property they have of _appearing_ to be evidence. As Logic has been here treated as embracing the whole reasoning process, so it must notice the fallacies incident to any part of it (not to Ratiocination merely), whether arising from faulty Induction, or from faulty Ratiocination, or from dispensing wholly with either or both of them. It does not treat of errors from negligence, or from inexpertness in using right methods, nor does it treat of errors from moral causes, viz. Indifference to truth, or Bias by our wishes or our fears; for the moral causes are but the _remote_ and _predisposing_, not the _exciting_ causes of opinions; and therefore inferences from them, since they must always involve the intellectual operation of admitting insufficient evidence as sufficient, really come under a classification of the things which wrongly _appear_ evidence to the _understanding_. Fallacies may be arranged, with reference either to the cause which makes them (erroneously) appear evidence, or to the particular kind of evidence they simulate. The following classification is grounded on both these considerations jointly. CHAPTER II. CLASSIFICATION OF FALLACIES.