# Lessons From

"What the Tortoise said

to Achilles"

Lewis Carroll is best known by most people for his story

*Alice's Adventures in Wonderland.* But in the tortoise community, he is famous for writing "What the Tortoise said to Achilles."

Mr. Carroll was a very clever man.

Even though most people don't know this story well, the mathematicians still think about it. They draw diagrams, debate, and generally spend long hours thinking about the puzzle set by the tortoise.

I don't.

As a tortoise, I do understand the story. But this is not one of the secrets I'm allowed to tell.

I can, however, tell you a few things most humans miss when they consider what Lewis Carroll was trying to teach with this cautionary tale.

Of course, to really understand the lessons, you do have to read the story. And I will admit that it's quite a chore for the non-mathematical modern mind.

When you read an old tale like this, it helps to have a literary mind. In case your mind is not usually given to literary pursuits, I'll point out one literary convention. The tortoise has no name. That means that he stands for all of us. We are all as wise as he is.

Ready?

Here is

## "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 another 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!"

"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 prossessed

*pockets* in those days) an enormous note-book and a pencil. "Proceed! And speak

*slowly*, please.

*Short-hand* isn't invented yet!"

That beautiful First Proposition of 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.

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 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!"

"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 note-book 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 don't dispute

*that*, I imagine?"

"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 notebook?"

"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 note-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.

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 note-book, 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 re-named

*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!*"

## Lessons from

What the Tortoise Said to Achilles.

Mathematicians can argue the Logic all they want. I see other lessons in "What the Tortoise Said to Achilles." Here's the tortoise view of this story.

- People who sit on the back of a tortoise will be sorry.
- Helmets pinch the brain.
- Z
**always** comes before football. - Warriors do not impress tortoises.
- Putting someone else's ideas into notebooks doesn't solve the problem.
- A tortoise has more Logic in its smallest scute than a person has, well, anywhere!

Keep it slow and steady.

Public Domain Dedication

"What the Tortoise said to Achilles" was written by Lewis Carroll, and appeared in the April 1895 issue of *Mind*. It is now available in the Public Domain.

Fair Use Repository
Keep it slow and steady.

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