In 1688, John Locke received a letter from scientist William Molyneaux posing a curious philosophical riddle: Suppose a blind man learned to identify a cube and a sphere by touch. If the shapes were then laid before him and his vision restored, could he identify them by sight alone?

Locke responded, “Your ingenious problem will deserve to be published to the world,” and he included a formulation of the problem in the second edition of the Essay Concerning Human Understanding.

Three hundred years later, it’s still an open question. (Locke agreed with Molyneaux that the answer is probably no: “The blind man, at first sight, would not be able with certainty to say which was the globe, which the cube, whilst he only saw them; though he could unerringly name them by his touch, and certainly distinguish them by the difference of their figures felt.”)

When I think of a unicorn, what I am thinking of is certainly not nothing; if it were nothing, then, when I think of a griffin, I should also be thinking of nothing, and there would be no difference between thinking of a griffin and thinking of a unicorn. But there certainly is a difference; and what can the difference be except that in the one case what I am thinking of is a unicorn, and in the other a griffin? And if the unicorn is what I am thinking of, then there certainly must be a unicorn, in spite of the fact that unicorns are unreal. In other words, though in one sense of the words there certainly are no unicorns–that sense, namely, in which to assert that there are would be equivalent to asserting that unicorns are real–yet there must be some other sense in which there are such things; since, if there were not, we could not think of them.

— G.E. Moore, Philosophical Studies, 1922

It deals with a game that [Theodore] Roosevelt and I used to play at Sagamore Hill. After an evening of talk, perhaps about the fringes of knowledge, or some new possibility of climbing inside the minds and senses of animals, we would go out on the lawn, where we took turns at an amusing little astronomical rite. We searched until we found, with or without glasses, the faint, heavenly spot of light-mist beyond the lower left-hand corner of the Great Square of Pegasus, when one or the other of us would then recite:

That is the Spiral Galaxy of Andromeda.
It is as large as our Milky Way.
It is one of a hundred million galaxies.
It is 750,000 light-years away.
It consists of one hundred billion suns, each larger than our sun.

After an interval Colonel Roosevelt would grin at me and say: ‘Now I think we are small enough! Let’s go to bed.’

— William Beebe, The Book of Naturalists, 1944

To divide 8101265822784 by 8, move the 8 from the number’s head to its tail.

To multiply 1034482758620689655172413793 by 3, move the 3 from the tail to the head.

During the Russian revolution, the mathematical physicist Igor Tamm was seized by anti-communist vigilantes at a village near Odessa where he had gone to barter for food. They suspected he was an anti-Ukrainian communist agitator and dragged him off to their leader.

Asked what he did for a living, he said he was a mathematician. The sceptical gang leader began to finger the bullets and grenades slung round his neck. ‘All right,’ he said, ‘calculate the error when the Taylor series approximation to a function is truncated after n terms. Do this and you will go free. Fail and you will be shot.’ Tamm slowly calculated the answer in the dust with his quivering finger. When he had finished, the bandit cast his eye over the answer and waved him on his way.

Tamm won the 1958 Nobel prize for physics but he never did discover the identity of the unusual bandit leader.

— John Barrow, “It’s All Platonic Pi in the Sky,” The Times Educational Supplement, May 11, 1993

Suppose we illustrate. You put a ball on a billiard-table, and, holding the cue lengthwise from side to side of the table, push the ball across the cloth. Here, in a rough way, the ball represents the ship, the cue the wind, only, as there is no waste of energy, the ball travels at the same rate as the cue; evidently it cannot go any faster. Now, let us suppose that a groove is cut diagonally across the table, from one corner-pocket to the other, and that the ball rolls in the groove. Propelled in the same way as before, the ball will now travel along the groove (and along the cue) in the same time as the cue takes to move across the table. The groove is much longer than the width of the table, double as long, in fact. The ball, therefore, travels much faster than the cue which impels it, since it covers double the distance in the same time. Just so does the tacking ship sail faster than the wind.

— “Some Famous Paradoxes,” The Illustrated American, Nov. 1, 1890

A pleasant anecdote is told of Partridge, the celebrated almanac maker. In traveling on horseback into the country he stopped for his dinner at an inn, and afterward called for his horse that he might reach the next town, where he intended to sleep. ‘If you would take my advice, sir,’ said the ostler, as he was about to mount his horse, ‘you will stay where you are for the night, as you will surely be overtaken by a pelting rain.’ ‘Nonsense, nonsense,’ said the almanac maker, ‘there is sixpence for you, my honest fellow, and good afternoon to you.’ He proceeded on his journey, and sure enough he was well drenched in a heavy shower. Partridge was struck with the man’s prediction, and being always intent on the interest of his almanac, he rode back on the instant, and was received by the ostler with a broad grin. ‘Well, sir, you see I was right after all.’ ‘Yes, my lad, you have been so, and here is a crown for you, but I give it you on condition that you tell me how you knew of this rain.’ ‘To be sure, sir,’ replied the man; ‘why the truth is we have an almanac in our house called Partridge’s Almanac, and the fellow is such a notorious liar, that whenever he promises us a fine day we always know that it will be the direct contrary.’

— The Golden Rule, and Odd-Fellows’ Family Companion, Oct. 16, 1847