Engaged to give a talk at a university, logician Raymond Smullyan arrived half an hour early and wrote the following sentence on the blackboard, “to give the audience something to mull over”:

You have no reason to believe this sentence.

This, he reasoned, was a paradox. If you have no reason to believe the sentence, then what it states is really the case, which is certainly a good reason to believe it. But if you have a good reason to believe it, then it must be true … which means that you have no reason to believe it.

Half an hour later he came down the stairs to a packed audience. Spotting a bright-looking boy in the front row, he pointed to the sentence and asked him, “Do you believe that sentence?”

“Yes,” said the boy.

“What is your reason?”

“I don’t have any.”

Smullyan asked, “Then why do you believe it?”

The boy said, “Intuition.”

(Raymond Smullyan, “Self-Reference in All Its Glory!” conference “Self-Reference,” Copenhagen, Oct. 31-Nov. 2, 2002.)

The samurai crab, Heikea japonica, earns its nickname well: Its shell bears a startling resemblance to the face of an angry warrior. Some Japanese believe that these crabs are reincarnated samurai who, defeated at the Battle of Dan-no-ura, threw themselves into the sea, as described in the epic Tale of the Heike.

Biologist Julian Huxley put forward the idea that the “faces” were an example of artificial selection. He suggested that fishermen who caught crabs with particularly face-shaped carapaces, believing them to be reincarnated spirits, threw them back into the sea, permitting them to reproduce while their brothers were eaten.

But humans don’t eat these crabs, and in any case the “warrior” crabs exist even far from sites of human fishing. Really the crabs are an example of another, equally compelling phenomenon — pareidolia, our tendency to see significant patterns where none exist.

This is Charles Darwin’s first diagram of an evolutionary tree, from his First Notebook on Transmutation of Species. He drew it around July 1837, barely a month after he’d opened his first full transmutation notebook.

“Case must be that one generation should have as many living as now,” he wrote. “To do this and to have as many species in same genus (as is) requires extinction. Thus between A + B the immense gap of relation. C + B the finest gradation. B + D rather greater distinction. Thus genera would be formed. Bearing relation to ancient types with several extinct forms.”

At the top he’s written “I think.”

The Shepard tone is an auditory illusion: A succession of overlapping scales are played, each ascending, and each scale fades out as its successor fades in an octave lower. The resulting impression is of a climbing pitch that never “arrives” anywhere, a rising note that never gets higher.

Among many other applications, this sound was used for the Batpod in Christopher Nolan’s films The Dark Knight and The Dark Knight Rises — the vehicle seems constantly to accelerate without ever changing gear. “When played on a keyboard,” wrote sound designer Richard King, “it gives the illusion of greater and greater speed; the pod appears unstoppable.”

(Thanks, Nick.)

01/31/2022 UPDATE: Similarly, the Risset Rhythm seems to speed up:

(Thanks, Chris.)

Here’s something remarkable: This formula, discovered in 1995 by David Bailey, Peter Borwein, and Simon Plouffe of the University of Quebec at Montreal, permits the calculation of isolated digits of π — it’s possible to calculate, say, the trillionth digit of π without working out all the preceding digits.

The catch is that it works only in base 2 (binary) and base 16 (hexadecimal), but not in base 10. So it’s possible to know, say, that the five trillionth binary digit of π is 0, but there’s no way to convert the result into its decimal equivalent without working out all the intervening binary digits.

“The new formula allows the calculation of the nth base 2 or base 16 digit of π in a time that is essentially linear in n, with memory requirements that grow logarithmically (very slowly) in n,” writes David Darling in The Universal Book of Mathematics. “One possible use of the Bailey-Borwein-Plouffe formula is to help shed light on whether the distribution of π’s digits are truly random, as most mathematicians suppose.”

08/14/2022 UPDATE: A new formula permits the extraction of decimal digits. (Thanks, Edward.)

A puzzle from reader Steven Moore:

Find A, B, and C as distinct integers. There is only one solution.

From Lee Sallows:

The traditional magic square is a square array of n×n distinct numbers, their magical property being that the sum of the n numbers occupying each row, column, and diagonal is the same. A variation on this theme that I introduced in 2011 is the geometric magic square in which distinct geometrical figures (usually planar shapes) occupy the cells of the array rather than numbers. The magical property enjoyed by such an array is then that the n shapes making up each row, column, and diagonal can be fitted together as in a jigsaw puzzle so as to yield (i.e. tile) a new compound shape that is the same in each case.

Beyond ‘ordinary’ geometric magic squares, it turns out that the combinative properties of shapes are such as to enable ‘magical’ constructions that are denied to analogous structures using numbers. For example, at left in the figure above is seen a 3×3 square of a kind that cannot be realized using distinct numbers rather than shapes. Note first that the square is to be understood as ‘toroidally-connected’, which is to say, as if inscribed on a torus. Its left-hand edge is then to be interpreted as adjacent to its right-hand edge and its top edge adjacent to its bottom edge. Its magical property is then that the four pieces contained within any 2×2 subsquare can be assembled to produce an identical shape, in this case a rectangle of size 4×5. In all there are nine such subsquares to be found in the square, as seen (again in a square) at right. Note that three of the pieces are disjoint, my attempts to produce a similar solution using nine unbroken pieces having failed. So whereas a 3×3 magic square, numerical or geometric, satisfies at least 8 separate conditions ( 3 rows + 3 columns + 2 diagonals), the square here shown satisfies one more.

(Thanks, Lee!)

Just a little detail that I thought was interesting: Famously the moon appears larger when it’s near the horizon, but you can defeat this illusion by bending over and viewing it between your legs.

Why this works isn’t clear — possibly it’s because the image is inverted on the retina, or it may be an effect of inverting the body’s orientation.

(Stanley Coren, “The Moon Illusion: A Different View Through the Legs,” Perceptual and Motor Skills 75:3 [1992], 827-831; Atsuki Higashiyama and Kohei Adachi, “Perceived Size and Perceived Distance of Targets Viewed From Between the Legs: Evidence for Proprioceptive Theory,” Vision Research 46:23 [2006], 3961-3976.)