Practical Logic


Chapter I


"All men naturally desire knowledge." This book is meant to minister to this passion primarily and secondarily to all interests that knowledge subserves.

Here will be found maxims for estimating the validity and strength of arguments, and for deciding what facts ought to be examined in the investigation of a question.

That the student may attain a real mastery of the art of thinking, it is necessary that the reasons for these maxims should be made clear to him, and that the maxims themselves should be woven into a harmonious code so as to be readily grasped by the mind.

Logic or dialectic is the name of the science from which such rules are drawn. For right reasoning has evidently been the object of inquiry for Aristotle in all the books of the Organon except perhaps the first, as it was also that of the Stoics, of the Lawyers, of the medieval Summulists, and of modern students of Induction, in the additions which they have made to the doctrines of the Stagyrite. "Dialectica," says the most celebrated medieval logic, "est ars artium, scientia scientiarum, ad omnium methodorum principia viam habens. Sola enim dialectica probabiliter disputat de principiis omnium aliarum scientiarum."

Exercise 1. Let the student write out an impartial discussion of the question whether the principles of right reasoning can be investigated. For it would seem that these principles must be known before any investigation whatever can be made. In this writing, let precision of thought be the first object, precision in the order of discussion the next. Let no ornament of style be permitted.

A science by which things are tested is necessarily a classificatory science. Thus, every system of qualitative chemical analysis consists in a classification of chemical substances. Accordingly, we have to study, in the first place, the classification of inferences. Just as there are several different systems of qualitative analysis,--as ordinary analysis by sulphuretted hydrogen, blowpipe analysis, and analysis by carbonate of baryta,--based on different classifications of chemical substances, but all valid, so there are different valid systems of logic, based on different classifications of inferences. The accomplished reasoner will do well to be familiar with more than one such system.


Chapter 2


First of all, the student has to gain a perfectly definite conception of the true function of reasoning.

The following axiom requires no comment, beyond the remark that it seems often to be forgotten. Where there is no real doubt or disagreement there is no question and can be no real investigation.

Upon the next point, somewhat more thought must be bestowed. Any useful inquisition must lead to some definite conclusion. A method of investigation which should carry different men to different results without tending to bring them to agreement, would be self-destructive and worthless. But if by a sufficiently long result a settlement of opinion could be reached, this concordance (even if further exploration would disturb it) is all that research really tends towards, and is therefore its only attainable end. The only legitimate aim of reasoning, then, is to ascertain what decision would be agreed upon if the question were sufficiently ventilated. To this it may be objected, 1st, that the primary object of an investigation is to ascertain the truth itself and not the opinions which would arise under any particular circumstances; and, 2nd, that the resolution of my own doubt is more my object in an investigation than the production of unanimity among others. Undoubtedly, that which we seek in an investigation is called truth, but what distinct conception ought to be attached to this word it is so difficult to say, that it seems better to describe the object of an investigation by a character which certainly belongs to it and to it alone, and which has nothing mysterious or vague about it. In like manner, it may be admitted that a genuine investigation is undertaken to resolve the doubts of the investigator. But observe this: no sensible man will be void of doubt as long as persons as competent to judge as himself differ from him. Hence to resolve his own doubts is to ascertain to what position sufficient research would carry all men.

For attaining this unanimous accord,--this catholic confession,--two plans have been pursued.

The first, simplest, and most usual is to adhere pertinaciously to some opinion and endeavour to unite all men upon it. The means of bringing men to agree to such a fixed opinion are an efficient organization of men who will devote themselves to propagating it, working upon the passions of mankind, and gaining an ascendency over them by keeping them in ignorance. In order to guard against all temptation to abandon his opinion, a man must be careful what he reads and must learn to regard his belief as holy, to be indignant at any questioning of it, and especially to consider the senses as the chief means whereby Satan gains access to the soul and as organs constantly to be mortified, distrusted, and despised. With an unwavering determination thus to shut himself off from all influences external to the society of those who think with him, a man may root //opinions/faith// in himself ineradicably; and a considerable body of such men, devoting all their energies to the spread of their doctrines, may produce a great effect under favourable circumstances. They and their followers may truly be said to be not of this world. Their actions will often be inexplicable to the rest of mankind, since they live in a world, which they will call spiritual and others will call imaginary, with reference to which their opinions are certainly perfectly true. The belief of one of these men, though perhaps resulting in large measure from the force of circumstances, will also be strengthened by a direct effort of the will, and he should therefore consistently regard it as wrong-willed and wicked to allow one's opinion to be formed, independently of what one wishes to believe, by that play of Sense which the Devil puts in one's way.

This method (which we may term the Divine, Spiritual, or Heavenly method) will not serve the purpose of the Children of This World, since the world in which they are interested has this peculiarity: that things are not just as we choose to think them. Consequently, the accord of those whose belief is determined by a direct effort of the will, is not the unanimity which these persons seek.