## Carroll’s Symbolic Logic

### March 2nd, 2009

Eight diagrams by Lewis Carroll (of Alice’s Adventures in Wonderland) from his 1897 Symbolic Logic, in which Carroll presents a symbolic method of representing propositions and visually deriving the logical outcomes of syllogisms, fallacies, and soriteses.

“First, let us suppose that the above [Biliteral] Diagram is an enclosure assigned to a certain Class of Things, which we have selected as our ‘Universe of Discourse,’ or, more briefly, as our ‘Univ’.

“Secondly, let us suppose that we have selected a certain Adjunct, which we may call *x*, and have divided the large Class, to which we have assigned the whole Diagram, into the two smaller Classes whose Differentiæ are *x* and not-*x* (which we may call *x’*), and that we have assigned the *North* Half of the Diagram to the one (which we may call… the *x*-Class), and the *South* Half to the other (which we may call… the *x’*-Class).

“Thirdly, let us suppose that we have selected another Adjunct, which we may call *y*, and have subdivided the *x*-Class into the two Classes whose Differentiæ are *y* and not-*y*, and that we have assigned the North-*West* Cell to the one (which we may call the *xy*-Class) and the North-*East* Cell to the other (which we may call the *xy’*-Class).

“Fourthly, let us suppose that we have subdivided the *x’*-Class in the same manner, and have assigned the South-*West* Cell to the *x’y*-Class, and the South-*East* Cell to the *x’y’*-Class” (p22-23).

For example, if *x* means ‘old’, so that *x’* means ‘new’, and if *y* means ‘English’, so that *y’* means ‘foreign’, we have the following diagram:

“Let us agree that a *Red* Counter [represented below as a dotted circle or as an ‘I’], placed within a Cell, shall mean ‘This Cell is *occupied*‘ (i.e. ‘There is at least *one* Thing in it’).

“Let us also agree that a *Red* Counter, placed on the partition between the two Cells, shall mean ‘The Compartment, made up of these two Cells, is *occupied*; but it is not known *whereabouts*, in it, its occupants are.’ Hence it may be understood to mean ‘At least *one* of these two Cells is occupied: possibly *both* are.

“Let us also agree that a *Grey* Counter [represented below as a hollow circle or as an ‘O’], placed within a Cell, shall mean ‘This Cell is *empty*‘ (i.e. ‘There is *nothing* in it’)” (p26).

By this formula we can visually represent the following propositions (p34, 35):

“The Reader should now get his genial friend to question him, severely, on [the above] two Tables” (p34).

Let us now support, firstly, “that we change [the Biliteral Diagram] into a *Triliteral* Diagram by drawing an *Inner Square*, so as to divide each of its 4 Cells into 2 portions, thus making 8 Cells altogether” (p39).

“Secondly, let us suppose that we have selected a certain Adjunct, which we may call *m*, and have subdivided the *xy*-Class into the two classes whose Differentiæ are *m* and *m’*, and that we have assigned the N.W. *Inner* Cell to the one (which we may call… the *xym*-Class)” (p40) and that we have subdivided the remaining classes in the same manner. “It is evident that we have now assigned the *Inner Square* to the *m*-Class, and the *Outer Border* to the *m’*-Class” (p40).

Thus we can visually represent propositions of the form (p49):

It is now possible to draw two propositions on the same diagram, one in terms of *x* and *m*, the other in terms of *m* and *y*, and to visually derive a third proposition in terms of *x* and *y*. Consider, for example, the following pair of propositions:

“Some, who deserve the fair, get their deserts;

None but the brave deserve the fair” (p101).

If the “Univ is ‘persons’; *m* = persons who deserve the fair; *x* = persons who get their deserts; *y* = brave” (p140), then the Triliteral Diagram represents that “some *m* are *x* [and] no *y’* are *m*”:

The Biliteral Diagram to the right, relating *x* and *y*, can be derived from the Triliteral Diagram according to the following procedure: for each quarter of the Triliteral Diagram, “if it contains a ‘I’ in *either* Cell, it is certainly *occupied*, and you may mark the… [corresponding] quarter of the Biliteral Diagram with a ‘I’. If it contains *two* ‘O’s, one in *each* Cell, it is certainly *empty*, and you may mark the… Biliteral Diagram with a ‘O'” (p53).

Therefore the derived Biliteral Diagram above concludes that “some brave persons get their deserts” (p140).

This method may also be used to validate the correctness, or expose the fallaciousness, of a proposed syllogism. For example:

“Some epicures are ungenerous;

All my uncles are generous

[Therefore] My uncles are not epicures” (107).

If “Univ. ‘persons’; *m* = generous ; *x* = epicures ; *y* = my uncles” (p145):

“Hence [the] proposed Conclusion is wrong, the right one being ‘Some epicures are not uncles of mine'” (p145).

Writes Carroll, “Mental recreation is a thing that we all of us need for our mental health; and you may get much healthy enjoyment, no doubt, from Games, such as Back-gammon, Chess, and new Game ‘Halma’. But, after all, when you have made yourself a first-rate player at any one of these Games, you have nothing real to *show* for it, as a *result*! You enjoyed the Game, and the victory, no doubt, *at the time*: but you have no *result* that you can treasure up and get real *good* out of. And, all the while, you have been leaving unexplored a perfect *mine* of wealth. Once master the machinery of Symbolic Logic, and you have a mental occupation always at hand, of absorbing interest, and one that will be of real *use* to you in *any* subject you may take up. It will give you clearness of thought — the ability to *see your way* through a puzzle — the habit of arranging your ideas in an orderly and get-at-able form — and, more valuable than all, the power to detect *fallacies*, and to tear to pieces the flimsy illogical arguments, which you will so continually encounter in books, in newspapers, in speeches, and even in sermons, and which so easily delude those who have never taken the trouble to master this fascinating Art. *Try it*. That is all I ask of you!” (pXVII).

The reader may pursue the outcome of the following two propositions on his or her own time:

“Nothing intelligible ever puzzles *me*;

Logic puzzles me” (p102).

April 27th, 2009 at 7:47 am

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