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ing what things are to be excluded from the number of possible forms. After this process of exclusion is accomplished, we are then possessed of a group of affirmative facts falling under the same predicament or class. This is the first step in the process of induction: it contracts the range of research, and brings the true explanation of the case in question within narrower limits. Thus, (to adopt the illustration adduced by Professor Playfair,) if the quality which is the cause of transparency in bodies be the subject of inquiry; from the fact that the diamond is transparent, we immediately exclude rarity or porosity, as well as fluidity; because the diamond is a very solid and dense body.*
Negative or opposing instances, i, e. facts where the given quality is wanting, must also, says Bacon, be carefully examined; for they are equally instructive. If, for instance, we confine air with moistened iron filings, in
* Playfair's Prelim. Diss. to Ency. Brit. p. 460.
a close vessel over water, its bulk will be diminished, owing to a portion of it having become combined with the iron in the form of rust; and, on examining the residual air, we shall find that it cannot support flame or animal life. Hence, from this negative or opposing fact, we discover that the cause of the support of flame and animal life is to be sought for in that part of the air which the iron has abstracted, viz. oxygen.
Comparative instances, viz. those which show the given quality in a certain order or scale of intensity, are likewise to be considered. This is the more necessary because unless this class of facts is particularly attended to, we shall be apt to conclude that cases which differ only in the degree of intensity are distinguished by a real opposition of quality. Transparency and opacity, for example, would at first sight appear in direct opposition; but, on considering the
* Herschel's Nat. Phil. § 151.
gradual diminution of transparency in natural substances, we shall see reason, (as sir John Herschel observes, to admit that the latter quality, instead of being the opposite of the former, is only its extreme lowest degree.'
To render the affirmative, negative, and comparative facts more available, lord Bacon suggests that a tabular arrangement of them should be adopted. It is in this part of the Novum Organum that the author proceeds to exemplify his inductive method with reference to Heat. His collection of facts is allowed by Bacon's warmest admirers to be imperfect, yet, in the opinion of Professor Playfair,—his ablest and most eloquent commentator,—it is extremely judicious, and the whole disquisition highly interesting.
Having, therefore, cautiously examined all the assembled data, --whether affirmative, negative, or comparative, we must then,
* Herschel's Nat. Phil. § 135.
after numerous rejections and exclusions,' (as before observed,*) elicit some few principles, common to every case; and having verified these, by trying if they will account for the phenomena which they represent, we must, by the same process of rejections and exclusions,' endeavour to reduce the principles which we have already obtained to some or one (if possible) more general. When we can advance no higher, the ultimate axiom must be assumed as the cause, and then verified in the same manner as the subordinate principles. If it account for all the phenomena, then we have rightly interpreted nature; and the law which was the object of our search, is evolved.
Some learned writers have supposed that Bacon's precepts relate only to physics; but the author himself designed them for all other sciences; nor does there appear any ground for asserting that the inductive me
* Ante p. 146.--For conclusions' read exclusions.
thod is not as applicable to the investigation of the laws of mind as the laws of matter. • Some may doubt,' says Bacon, whether we propose to apply our method of investigation to natural philosophy only, or to other sciences, such as logic, ethics, politics? We answer, that we mean it to be so applied. And as the common logic, which proceeds by the syllogism, belongs not only to natural philosophy, but to all the sciences, so our logic, which proceeds by induction, embraces every thing.'
As a further help to inductions, lord Bacon proposes that, among the mass of facts brought in for examination, those which strike us as peculiarly fitted for our purpose should hold a higher rank in the table of instances. These characteristic phenomena he accordingly terms Prerogativa Instantiarum, enumerating twenty-seven different species;
* Novum Organum, lib. i. aph. 127; Playfair's Prelim. Diss. p. 460; and see Ed. Phil. Trans. vol. 8, p 382.