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illusory 373 5. The cause is not the invariable antecedent, but the unconditional invariable antecedent 375 6. Can a cause be simultaneous with its effect? 380 7. Idea of a Permanent Cause, or original natural agent 383 8. Uniformities of coexistence between effects of different permanent causes, are not laws 386 9. Doctrine that volition is an efficient cause, examined 387
Chapter VI. Of the Composition of Causes. § 1. Two modes of the conjunct action of causes, the mechanical and the chemical 405 2. The composition of causes the general rule; the other case exceptional 408 3. Are effects proportional to their causes? 412
Chapter VII. Of Observation and Experiment. § 1. The first step of inductive inquiry is a mental analysis of complex phenomena into their elements 414 2. The next is an actual separation of those elements 416 3. Advantages of experiment over observation 417 4. Advantages of observation over experiment 420
Chapter VIII. Of the Four Methods of Experimental Inquiry. § 1. Method of Agreement 425 2. Method of Difference 428 3. Mutual relation of these two methods 429 4. Joint Method of Agreement and Difference 433 5. Method of Residues 436 6. Method of Concomitant Variations 437 7. Limitations of this last method 443
Chapter IX. Miscellaneous Examples of the Four Methods. § 1. Liebig's theory of metallic poisons 449 2. Theory of induced electricity 453 3. Dr. Wells' theory of dew 457 4. Dr. Brown-Séquard's theory of cadaveric rigidity 465 5. Examples of the Method of Residues 471 6. Dr. Whewell's objections to the Four Methods 475
Chapter X. Of Plurality of Causes; and of the Intermixture of Effects. § 1. One effect may have several causes 482 2. —which is the source of a characteristic imperfection of the Method of Agreement 483 3. Plurality of Causes, how ascertained 487 4. Concurrence of Causes which do not compound their effects 489 5. Difficulties of the investigation, when causes compound their effects 494 6. Three modes of investigating the laws of complex effects 499 7. The method of simple observation inapplicable 500 8. The purely experimental method inapplicable 501
Chapter XI. Of the Deductive Method. § 1. First stage; ascertainment of the laws of the separate causes by direct induction 507 2. Second stage; ratiocination from the simple laws of the complex cases 512 3. Third stage; verification by specific experience 514
Chapter XII. Of the Explanation of Laws of Nature. § 1. Explanation defined 518 2. First mode of explanation, by resolving the law of a complex effect into the laws of the concurrent causes and the fact of their coexistence 518 3. Second mode; by the detection of an intermediate link in the sequence 519 4. Laws are always resolved into laws more general than themselves 520 5. Third mode; the subsumption of less general laws under a more general one 524 6. What the explanation of a law of nature amounts to 526
Chapter XIII. Miscellaneous Examples of the Explanation of Laws of Nature. § 1. The general theories of the sciences 529 2. Examples from chemical speculations 531 3. Example from Dr. Brown-Séquard's researches on the nervous system 533 4. Examples of following newly-discovered laws into their complex manifestations 534 5. Examples of empirical generalizations, afterwards confirmed and explained deductively 536 6. Example from mental science 538 7. Tendency of all the sciences to become deductive 539

INTRODUCTION.

§ 1. There is as great diversity among authors in the modes which they have adopted of defining logic, as in their treatment of the details of it. This is what might naturally be expected on any subject on which writers have availed themselves of the same language as a means of delivering different ideas. Ethics and jurisprudence are liable to the remark in common with logic. Almost every writer having taken a different view of some of the particulars which these branches of knowledge are usually understood to include; each has so framed his definition as to indicate beforehand his own peculiar tenets, and sometimes to beg the question in their favour.

This diversity is not so much an evil to be complained of, as an inevitable and in some degree a proper result of the imperfect state of those sciences. It is not to be expected that there should be agreement about the definition of anything, until there is agreement about the thing itself. To define, is to select from among all the properties of a thing, those which shall be understood to be designated and declared by its name; and the properties must be well known to us before we can be competent to determine which of them are fittest to be chosen for this purpose. Accordingly, in the case of so complex an aggregation of particulars as are comprehended in anything which can be called a science, the definition we set out with is seldom that which a more extensive knowledge of the subject shows to be the most appropriate. Until we know the particulars themselves, we cannot fix upon the most correct and compact mode of circumscribing them by a general description. It was not until after an extensive and accurate acquaintance with the details of chemical phenomena, that it was found possible to frame a rational definition of chemistry; and the definition of the science of life and organization is still a matter of dispute. So long as the sciences are imperfect, the definitions must partake of their imperfection; and if the former are progressive, the latter ought to be so too. As much, therefore, as is to be expected from a definition placed at the commencement of a subject, is that it should define the scope of our inquiries: and the definition which I am about to offer of the science of logic, pretends to nothing more, than to be a statement of the question which I have put to myself, and which this book is an attempt to resolve. The reader is at liberty to object to it as a definition of logic; but it is at all events a correct definition of the subject of these volumes.

§ 2. Logic has often been called the Art of Reasoning. A writer[1] who has done more than any other person to restore this study to the rank from which it had fallen in the estimation of the cultivated class in our own country, has adopted the above definition with an amendment; he has defined Logic to be the Science, as well as the Art, of reasoning; meaning by the former term, the analysis of the mental process which takes place whenever we reason, and by the latter, the rules, grounded on that analysis, for conducting the process correctly. There can be no doubt as to the propriety of the emendation. A right understanding of the mental process itself, of the conditions it depends on, and the steps of which it consists, is the only basis on which a system of rules, fitted for the direction of the process, can possibly be founded. Art necessarily presupposes knowledge; art, in any but its infant state, presupposes scientific knowledge: and if every art does not bear the name of a science, it is only because several sciences are often necessary to form the groundwork of a single art. So complicated are the conditions which govern our practical agency, that to enable one thing to be done, it is often requisite to know the nature and properties of many things.

Logic, then, comprises the science of reasoning, as well as an art, founded on that science. But the word Reasoning, again, like most other scientific terms in popular use, abounds in ambiguities. In one of its acceptations, it means syllogizing; or the mode of inference which may be called (with sufficient accuracy for the present purpose) concluding from generals to particulars. In another of its senses, to reason is simply to infer any assertion, from assertions already admitted: and in this sense induction is as much entitled to be called reasoning as the demonstrations of geometry.

Writers on logic have generally preferred the former acceptation of the term: the latter, and more extensive signification is that in which I mean to use it. I do this by virtue of the right I claim for every author, to give whatever provisional definition he pleases of his own subject. But sufficient reasons will, I believe, unfold themselves as we advance, why this should be not only the provisional but the final definition. It involves, at all events, no arbitrary change in the meaning of the word; for, with the general usage of the English language, the wider signification, I believe, accords better than the more restricted one.

§ 3. But Reasoning, even in the widest sense of which the word is susceptible, does not seem to comprehend all that is included, either in the best, or even in the most current, conception of the scope and province of our science. The employment of the word Logic to denote the theory of argumentation, is derived from the Aristotelian, or, as they are commonly termed, the scholastic, logicians. Yet even with them, in their systematic treatises, argumentation was the subject only of the third part: the two former treated

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