The Calculus of Logic by George Boole, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. Laws of Syllogism deduced from the Elective Calculus.
Laws of Syllogism deduced from the Elective Calculus.
We shall take into account all propositions which can be made out of the classes X, Y, Z, and referred to any of the forms embraced in the following system.
It is necessary to recapitulate that quantity (universal and particular) and quality (affirmative and negative) are understood to belong to the terms of propositions which is indeed the correct view.[8]
Thus, in the proposition All Xs are Ys, the subject All Xs is universal-affirmative, the predicate (some) Ys particular-affirmative.
In the proposition, some Xs are Zs, both terms are particular-affirmative.
The proposition No Xs are Zs would in philosophical language be written in the form All Xs are not-Zs. The subject is universal-affirmative, the predicate particular-negative.
In the proposition Some Xs are not-Zs are Ys the subject is universal-negative, the predicate particular-affirmative, and so on.
In a pair of premises there are four terms, viz. two subjects and two predicates; two of these terms, viz. those involving the Y or not-Y may be called the middle terms, the two others the extremes, one of these involving X or not-X, the other Z or not-Z.
The following are then the conditions and the rules of inference.
Case 1st. The middle terms of like quality.
Condition of Inference. One middle term universal.
Rule. Equate the extremes.
Case 2nd. The middle terms of opposite qualities.
1st. Condition of Inference. One extreme universal.
Rule. Change the quantity and quality of that extreme, and equate the result to the other extreme.
2nd. Condition of inference. Two universal middle terms.
Rule. Change the quantity and quality of either extreme, and equate the result to the other extreme.
I add a few examples,
|
1st. |
All Ys are Xs |
|---|---|
|
|
All Zs are Ys. |
This belongs to Case 1. All Ys is the universal middle term. The extremes equated give All Zs are Xs, the stronger term becoming the subject.
This belongs to Case 2, and satisfies the first condition. The middle term is particular-affirmative in the first premise, particular-negative in the second. Taking All Zs as the universal extreme, we have, on changing its quantity and quality, Some not-Zs, and this equated to the other extreme gives
All Xs are (some) not-Zs = No Xs are Zs.
If we take All Xs as the universal extreme we get
No Zs are Xs
3rd. All Xs are Ys.
Some Zs are not-Ys.
This also belongs to Case 2, and satisfies the first condition. The universal extreme All Xs becomes, some not-Xs, whence
Some Zs are not-Xs.
4th. All Ys are Xs.
All not-Ys are Zs.
This belongs to Case 2, and satisfies the second condition. The extreme Some Xs becomes All not-Xs,
The other extreme treated in the same way would give
All not-Zs are Xs,
which is an equivalent result.
If we confine ourselves to the Aristotelian premises A, E, I, O, the second condition of inference in Case 2 is not needed. The conclusion will not necessarily be confined to the Aristotelian system.
This belongs to Case 2, and satisfies the first condition. The result is
Some not-Zs are not-Xs.
These appear to me to be the ultimate laws of syllogistic inference. They apply to every case, and they completely abolish the distinction of figure, the necessity of conversion, the arbitrary and partial[9] rules of distribution, &c. If all logic were reducible to the syllogism these might claim to be regarded as the rules of logic. But logic, considered as the science of the relations of classes has been shewn to be of far greater extent. Syllogistic inference, in the elective system, corresponds to elimination. But this is not the highest in the order of its processes. All questions of elimination may in that system be regarded as subsidiary to the more general problem of the solution of elective equations. To this problem all questions of logic and of reasoning, without exception, may be referred. For the fuller illustrations of this principle I must however refer to the original work. The theory of hypothetical propositions, the analysis of the positive and negative elements, into which all propositions are ultimately resolvable, and other similar topics are also there discussed.
Undoubtedly the final aim of speculative logic is to assign the conditions which render reasoning possible, and the laws which determine its character and expression. The general axiom (A) and the laws (1), (2), (3), appear to convey the most definite solution that can at present be given to this question. When we pass to the consideration of hypothetical propositions, the same laws and the same general axiom which ought perhaps also to be regarded as a law, continue to prevail; the only difference being that the subjects of thought are no longer classes of objects, but cases of the coexistent truth or falsehood of propositions. Those relations which logicians designate by the terms conditional, disjunctive, &c., are referred by Kant to distinct conditions of thought. But it is a very remarkable fact, that the expressions of such relations can be deduced the one from the other by mere analytical process. From the equation y = vx, which expresses theconditional proposition, "If the proposition Y is true the proposition X is true.” we can deduce
which expresses the disjunctive proposition, "Either Y and X are together true, or X is true and Y is false, or they are both false,” and again the equation y(1 - x) = 0, which expresses a relation of coexistence, viz. that the truth of Y and the falsehood of X do not coexist. The distinction in the mental regard, which has the best title to be regarded as fundamental, is, I conceive, that of the affirmative and the negative. From this we deduce the direct and the inverse in operations, the true and the false in propositions, and the opposition of qualities in their terms.
The view which these enquiries present of the nature of language is a very interesting one. They exhibit it not as a mere collection of signs, but as a system of expression, the elements of which are subject to the laws of the thought which they represent. That those laws are as rigorously mathematical as are the laws which govern the purely quantitative conceptions of space and time, of number and magnitude, is a conclusion which I do not hesitate to submit to the exactest scrutiny.
[8] When propositions are said to be affected with quantity and quality, the quality is really that of the predicate, which expresses the nature of the assertion, and the quantity that of the subject, which shews its extent.
[9] Partial, because they have reference only to the quantity of the X, even when the proposition relates to the not-X. It would be possible to construct an exact counterpart to the Aristotelian rules of syllogism, by quantifying only the not-X. The system in the text is symmetrical because it is complete.
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