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An Introduction to Lewis Carroll and Logic

An Introduction to Lewis Carroll and Logic. Kara Levis Spring 2009 Period 3. Charles Lutwidge Dodgson. Lewis Carroll used the concepts of formal logic as theorized by the Ancient Greeks

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An Introduction to Lewis Carroll and Logic

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  1. An Introduction to Lewis Carroll and Logic Kara Levis Spring 2009 Period 3

  2. Charles Lutwidge Dodgson • Lewis Carroll used the concepts of formal logic as theorized by the Ancient Greeks • The following slides will detail the uses of reason in solving a class of problems called syllogisms, in which a set of statements are used to arrive at an inevitable conclusion

  3. Simple Syllogisms • The easiest syllogisms are written as three statements: • All babies are illogical • Nobody is despised who can manage a crocodile • Illogical persons are despised • Obviously the problem need not make sense, however, from this mess there is a single conclusion that can be drawn when the given statements are studied

  4. Simple Syllogisms • The statements can be arranged as follows, termed in the format of “if…, then…”: • All babies are illogical • If it is a baby, then it is illogical • BI • Nobody is despised who can manage a crocodile • If it is despised, then it cannot manage a crocodile • D~C • Illogical persons are despised • If it is illogical, then it is despised • ID • Here, B=it is a baby, I=it is illogical, D=it is despised, and C=it can manage a crocodile • The “~” symbol negates the term to which it applies • It is assumed that all statements can be reversed if necessary

  5. Simple Syllogisms • By stringing together the phrases in the previous slide, we get a process similar to the transitive property in which if a=b, and b=c, then a=c • BI, D~C, ID • BID~C • This would be phrased “If it is a baby, then it is illogical. If it is illogical, the it is despised. If it is despised, then it cannot manage a crocodile” • This leads to the ultimate conclusion that B~C, essentially that “If it is a baby, then it cannot manage a crocodile” • In more normal terms, the conclusion is that “No baby can manage a crocodile”

  6. Shortcut • There is a simple way to find the end statement • When writing the list of “if…, then…” phrases, find the two statements that do not have a pair within the set: • BI, D~C, ID • In this set, it is evident that neither “B,” nor “~C,” have a match within the list, even though “I,” and “D,” each appear twice • This signals that “B” and “~C” are the variables we are trying to connect in solving this syllogism

  7. Examples • Here are some simple syllogisms to try: • 1. Every one who is sane can do Logic.No lunatics are fit to serve on a jury.None of your sons can do logic. • 2. My saucepans are the only things I have that are made of tin.I find all your presents very useful.None of my saucepans are of the slightest use. • 3. No potatoes of mine, that are new, have been boiled.All my potatoes in this dish are fit to eat.No unboiled potatoes of mine are fit to eat.

  8. Answers • 1. • SL, ~S~J, Y~L • Y~L~S~J • “If it is your son, then it is not fit to serve on a jury” • “None of your sons are fit to serve on a jury” • 2. • TS, YU, S~U • YU~S~T • “If it is a present from you, then it is not made of tin” • “Your presents to me are not made of tin” • 3. • N~B, DE, ~B~E • DEB~N • “If it is a potato in this dish, then it is not new” • “No potatoes in this dish are new”

  9. Complex Syllogisms • Once you get the hang of simple syllogisms, there are more complex versions with more than three steps • These are solved the same way as three-step syllogisms, but can become confusing • Example: • None of the unnoticed things, met with at sea, are mermaids. • Things entered in the log, as met with at sea, are sure to be worth remembering. • I have never met with anything worth remembering, when on a voyage. • Things met with at sea, that are noticed, are sure to be recorded in the log

  10. Complex Syllogisms • For the example, the following is true: • MerN, LR, Met~R, NL • Met~R~L~N~Mer • “If I have met with it at sea, then it is not a mermaid” • “I have never met with a mermaid at sea”

  11. Carroll on Logic • However, despite the nature of his syllogisms, Carroll criticized certain ways of thinking: “What the Tortoise Said to Achilles” by Lewis Carroll Achilles had overtaken the Tortoise, and had seated himself comfortably on its back. “So you’ve got to the end of our race-course?” said the Tortoise. “Even though it DOES consist of an infinite series of distances? I thought some wiseacre or other had proved that the thing couldn’t be done?” “It CAN be done,” said Achilles. “It HAS been done! Solvitur ambulando. You see the distances were constantly DIMINISHING; and so –” “But if they had been constantly INCREASING?” the Tortoise interrupted. “How then?” “Then I shouldn’t be here,” Achilles modestly replied; “and YOU would have got several times round the world, by this time!”

  12. “What the Tortoise Said to Achilles” “You flatter me – FLATTEN, I mean,” said the Tortoise; “for you ARE a heavy weight, and NO mistake! Well now, would you like to hear of a race-course, that most people fancy they can get to the end of in two or three steps, while it REALLY consists of an infinite number of distances, each one longer than the previous one?” “Very much indeed!” said the Grecian warrior, as he drew from his helmet (few Grecian warriors possessed POCKETS in those days) an enormous note-book and pencil. “Proceed! And speak SLOWLY, please! SHORTHAND isn’t invented yet!” “That beautiful First Proposition by Euclid!” the Tortoise murmured dreamily. “You admire Euclid?” “Passionately! So far, at least, as one CAN admire a treatise that won’t be published for some centuries to come!” “Well, now, let’s take a little bit of the argument in that First Proposition – just TWO steps, and the conclusion drawn from them. Kindly enter them in your note-book. And in order to refer to them conveniently, let’s call them A, B, and Z: – (A) Things that are equal to the same are equal to each other.(B) The two sides of this Triangle are things that are equal to the same.(Z) The two sides of this Triangle are equal to each other.

  13. “What the Tortoise Said to Achilles” Readers of Euclid will grant, I suppose, that Z follows logically from A and B, so that any one who accepts A and B as true, MUST accept Z as true?” “Undoubtedly! The youngest child in a High School – as soon as High Schools are invented, which will not be till some two thousand years later – will grant THAT.” “And if some reader had NOT yet accepted A and B as true, he might still accept the SEQUENCE as a VALID one, I suppose?” “No doubt such a reader might exist. He might say, ‘I accept as true the Hypothetical Proposition that, if A and B be true, Z must be true; but I DON’T accept A and B as true.’ Such a reader would do wisely in abandoning Euclid, and taking to football.” “And might there not ALSO be some reader who would say ‘I accept A and B as true, but I DON’T accept the Hypothetical’?” “Certainly there might. HE, also, had better take to football.” “And NEITHER of these readers,” the Tortoise continued, “is AS YET under any logical necessity to accept Z as true?” “Quite so,” Achilles assented. “Well, now, I want you to consider ME as a reader of the SECOND kind, and to force me, logically, to accept Z as true.” “A tortoise playing football would be –” Achilles was beginning. “– an anomaly, of course,” the Tortoise hastily interrupted. “Don’t wander from the point. Let’s have Z first, and football afterwards!”

  14. “What the Tortoise Said to Achilles” “I’m to force you to accept Z, am I?” Achilles said musingly. “And your present position is that you accept A and B, but you DON’T accept the Hypothetical –” “Let’s call it C,” said the Tortoise. “– but you DON’T accept (C) If A and B are true, Z must be true.” “That is my present position,” said the Tortoise. “Then I must ask you to accept C.” “I’ll do so,” said the Tortoise, “as soon as you’ve entered it in that notebook of yours. What else have you got in it?” “Only a few memoranda,” said Achilles, nervously fluttering the leaves: “a few memoranda of – of the battles in which I have distinguished myself!” “Plenty of blank leaves, I see!” the Tortoise cheerily remarked. “We shall need them ALL!” (Achilles shuddered.) “Now write as I dictate: – (A) Things that are equal to the same are equal to each other.(B) The two sides of this Triangle are things that are equal to the same.(C) If A and B are true, Z must be true.(Z) The two sides of this Triangle are equal to each other. “You should call it D, not Z,” said Achilles. “It comes NEXT to the other three. If you accept A and B and C, you MUST accept Z.” “And why must I?” “Because it follows LOGICALLY from them. If A and B and C are true, Z MUST be true. You can’t dispute THAT, I imagine?”

  15. “What the Tortoise Said to Achilles” “If A and B and C are true, Z MUST be true,” the Tortoise thoughtfully repeated. “That’s ANOTHER Hypothetical, isn’t it? And, if I failed to see its truth, I might accept A and B and C, and STILL not accept Z, mightn’t I?” “You might,” the candid hero admitted; “though such obtuseness would certainly be phenomenal. Still, the event is POSSIBLE. So I must ask you to grant ONE more Hypothetical.” “Very good, I’m quite willing to grant it, as soon as you’ve written it down. We will call it (D) If A and B and C are true, Z must be true. Have you entered that in your note-book?” “I HAVE!” Achilles joyfully exclaimed, as he ran the pencil into its sheath. “And at last we’ve got to the end of this ideal race-course! Now that you accept A and B and C and D, OF COURSE you accept Z.” “Do I?” said the Tortoise innocently. “Let’s make that quite clear. I accept A and B and C and D. Suppose I STILL refused to accept Z?“ “Then Logic would take you by the throat, and FORCE you to do it!” Achilles triumphantly replied. “Logic would tell you, ‘You can’t help yourself. Now that you’ve accepted A and B and C and D, you MUST accept Z.’ So you’ve no choice, you see.” “Whatever LOGIC is good enough to tell me is worth WRITING DOWN,” said the Tortoise. “So enter it in your book, please. We will call it (E) If A and B and C and D are true, Z must be true. Until I’ve granted THAT, of course I needn’t grant Z. So it’s quite a NECESSARY step, you see?” “I see,” said Achilles; and there was a touch of sadness in his tone.

  16. “What the Tortoise Said to Achilles” Here the narrator, having pressing business at the Bank, was obliged to leave the happy pair, and did not again pass the spot until some months afterwards. When he did so, Achilles was still seated on the back of the much-enduring Tortoise, and was writing in his notebook, which appeared to be nearly full. The Tortoise was saying, “Have you got that last step written down? Unless I've lost count, that makes a thousand and one. There are several millions more to come. And WOULD you mind, as a personal favour, considering what a lot of instruction this colloquy of ours will provide for the Logicians of the Nineteenth Century – WOULD you mind adopting a pun that my cousin the Mock-Turtle will then make, and allowing yourself to be renamed TAUGHT-US?” “As you please,” replied the weary warrior, in the hollow tones of despair, as he buried his face in his hands. “Provided that YOU, for YOUR part, will adopt a pun the Mock-Turtle never made, and allow yourself to be re-named A KILL-EASE!”

  17. Carroll on Logic • This essay by Carroll demonstrates the point that nothing can be proven certainly until every hypothetical claim leading to the conclusion is proven to be reality • If one tries to apply this logic to Carroll’s syllogisms, the problems become nearly impossible • This is mainly because the syllogisms rely on deductive reasoning, in which a conclusion is found to be true following a list of true premises • Conversely, in inductive reasoning, the conclusion follows as a probable result of the premises, rather than a truth

  18. Reason • Carroll’s essay criticizes both types of reasoning, arguing that there are infinite steps necessary to reach between any set of premises and their suggested conclusion • If the premises are true, then one must prove that these premises lead to the suggested conclusion • This connecting statement must then be proven in the sequence of premises, and so on, giving an infinite set of steps between the premises and the conclusion

  19. The Last Point • Now that we’ve discussed Lewis Carroll and some of his opinions on logic, I will leave you with a problem to ponder… Enjoy this 10-step syllogism! The only animals in this house are cats.Every animal is suitable for a pet, that loves to gaze at the moon.When I detest an animal, I avoid it.No animals are carnivorous, unless they prowl at night.No cat fails to kill mice.No animals ever take to me, except what are in this house.Kangaroos are not suitable for pets.None but carnivora kill mice.I detest animals that do not take to me.Animals, that prowl at night, always love to gaze at the moon. • Try making up some problems for your friends… Math can be fun!

  20. Good Luck, and Happy Puzzling!

  21. My Own Puzzles • Simple: • Jungle creatures cannot be tamed • All monkeys live in the jungle • All good pets are tame • Complex: • When cranky, one cannot produce good work • No woman works in a factory • Inhaling smoke will cause lung cancer • Those with lung cancer are cranky • All factories are full of smoke • Slightly more complex: • I am a wet cat • When in Antarctica, one will always meet a penguin • Those who despise the cold are not easily amused • Anyone who meets a penguin will befriend it • Only Antarctica is isolated from the world • Friends of penguins are good people • Wet cats despise the cold • Those not easily amused will go to isolation

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