The principal rules for definition are three; viz. 1st. The definition must be adequate; i. e. neither too extensive nor too narrow for the thing defined: e. g. to define "fish," "an animal that lives in the water," would be too extensive, because many insects, &c. live in the water; to define it, "an animal that has an air-bladder," would be too narrow; because many fish are without any.

2d. The definition must be in itself plainer than the thing defined, else it would not explain it: I say, "in itself," (i. e. generally) because, to some particular person, the term defined may happen to be even more familiar and better understood, than the language of the definition.

3d. The Third Rule usually given by Logicians for a definition, is, that it should be couched in a convenient number of appropriate words (if such can be found suitable for the purpose) since figurative words (which are opposed to appropriate) are apt to produce ambiguity or indistinctness; too great brevity may occasion obscurity; and too great prolixity, confusion. But this perhaps is rather an admonition with respect to Style, than a strictly logical rule; nor can we accordingly determine with

this as it may, however, it is plain that the etymology of a term has nothing to do with any logical consideration of it. See note to § 8, of Book III.

precision, in each case, whether it has been complied with or not; there is no drawing the line between "too long" and "too concise," &c. Nor would a definition unnecessarily prolix be censured as incorrect, but as inelegant, inconvenient, &c. If, however, a definition be chargeable with Tautology, (which is a distinct fault from prolixity or verbosity) it is properly incorrect, though without offending against the first two rules. Tautology consists in inserting too much, not in mere words, but in sense; yet not so as too much to narrow the definition (in opposition to Rule 1.) by excluding some things which belong to the class of the thing defined; but only, so as to state something which has been already implied. Thus, to define a Parallelogram “a four-sided figure whose opposite sides are parallel and equal, would be tautological; because, though it is true that such a figure, and such alone, is a parallelogram, the equality of the sides is implied in their being parallel, and may be proved from it. Now the insertion of the words "and equal," leaves, and indeed leads, a reader to suppose that there may be a four-sided figure whose opposite sides are parallel but not equal.* Though therefore

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* This would be inferred according to the principle of exceptio probat regulum," an exception proves a rule. The force of the maxim is this; (for it is not properly

such a definition asserts nothing false, it leads to a supposition of what is false; and consequently is to be regarded as an incorrect definition.

confined to the case where an exception, strictly so called, is mentioned) that the mention of any circumstance introduced into the statement of a precept, law, remark, &c. (for the application of the maxim is not confined to the case of Definitions) is to be presumed necessary to be inserted; so that the precept, &c. would not hold good if this circumstance were absent. If e. g. it be laid down that he who breaks into an empty house shall receive a certain punishment, it would be inferred that this punishment would not be incurred by breaking into an occupied house : if it were told us that some celestial phenomenon could not be seen by the naked eye, it would be inferred that it would be visible through a telescope: &c.

And much is often inferred in this manner, which was by no means in the Author's mind; from his having inaccurately inserted what chanced to be present to his thoughts. Thus, he who says that it is a crime for people to violate the property of a humane Landlord who lives among them, may perhaps not mean to imply that it is no crime to violate the property of an absentee-landlord, or of one who is not humane; but he leaves an opening for being so understood. Thus again (to recur to the case of definitions) in saying that "an animal which breathes through gills and is scaly, is a fish," though nothing false is asserted, a presumption is afforded that you mean to give too narrow a definition; in violation of Rule I.

And Tautology, as above described, is sure to mislead any one who interprets what is said, conformably to the maxim that the exception proves a rule.

L

Definition of
Fallacy.

BOOK III.

OF FALLACIES.

Introduction.

By a Fallacy is commonly understood," any unsound mode of arguing, which appears to demand our conviction, and to be decisive of the question in hand, when in fairness it is not." Considering the ready detection and clear exposure of Fallacies to be both more extensively important, and also more difficult, than many are aware of, I propose to take a Logical view of the subject; referring the different Fallacies to the most convenient heads, and giving a scientific analysis of the procedure which takes place in each.

After all, indeed, 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: but still we shall find that to take correct general views of the subject, and to be familiarized with

scientific discussions of it, will tend, above all
things, to engender such a habit of mind, as
will best fit us for practice.

Indeed the case is the same with respect to
Logic in general; scarcely any one would, in
ordinary practice, state to himself either his
own or another's reasoning, in Syllogisms in
Barbara at full length; yet a familiarity with
Logical principles tends very much (as all
feel, who are really well acquainted with
them) to beget a habit of clear and sound
reasoning. The truth is, in this, as in many
other things, there are processes going on in
the mind (when we are practising anything
quite familiar to us) with such rapidity as to
leave no trace in the memory; and we often
apply principles which did not, as far as
we are conscious, even occur to us at the
time.

language of

ters.

It would be foreign, however, to the pre- Inaccurate sent purpose, to investigate fully the manner foriner wri in which certain studies operate in remotely producing certain effects on the mind: it is sufficient to establish the fact, that habits of ✓ scientific analysis (besides the intrinsic beauty and dignity of such studies) lead to practical advantage. It is on Logical principles therefore that I propose to discuss the subject of Fallacies; and it may, indeed, seem to have been unnecessary to make any apology for