James Watt perfects the steam engine, 1765:

I had gone to take a walk on a fine Sunday afternoon. I had entered the Green and had passed the old washing house. I was thinking up on the engine at the time and had got as far as the herd’s house, when the idea came into my mind that as steam was an elastic body it would rush into a vacuum, and that if a communication were made between the cylinder and an exhausted vessel it would rush into it and might there be condensed without cooling the cylinder. I had not walked farther than the golf house when the whole thing was arranged clearly in my mind.

Charles Darwin realizes why species diverge, 1840s:

I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me; and this was long after I had come to Down. The solution, as I believe, is that the modified offspring of all dominant and increasing forms tend to become adapted to many and highly diversified places in the economy of nature.

Henri Poincaré discovers the relation between automorphic functions and non-Euclidean geometries, 1881:

Just at this time, I left Caen, where I was living, to go on a geologic excursion under the auspices of the School of Mines. The incidents of the travel made me forget my mathematical work. Having reached Coutances, we entered an omnibus to go some place or other. At the moment when I put my foot on the step, the idea came to me, without anything in my former thoughts seeming to have paved the way for it, that the transformations I had used to define the Fuchsian functions were identical with those of non-Euclidian geometry. I did not verify the idea; I should not have had time, as, upon taking my seat in the omnibus, I went on with a conversation already commenced, but I felt a perfect certainty. On my return to Caen, for conscience’ sake, I verified the result at my leisure.

Walter Cannon recognizes the fight-or-flight response, 1911:

As a matter of routine I have long trusted unconscious processes to serve me. … [One] example I may cite was the interpretation of the significance of bodily changes which occur in great emotional excitement, such as fear and rage. These changes — the more rapid pulse, the deeper breathing, the increase in sugar in the blood, the secretion from the adrenal glands — were very diverse and seemed unrelated. Then, one wakeful night, after a considerable collection of these changes had been disclosed, the idea flashed through my mind that they could be nicely integrated if conceived as bodily preparations for supreme effort in flight or in fighting.

William Rowan Hamilton conceives the fundamental formula for quaternions, 1843:

But on the 16th day of the same month — which happened to be a Monday, and a Council day of the Royal Irish Academy — I was walking in to attend and preside, and your mother was walking with me, along the Royal Canal, to which she had perhaps driven; and although she talked with me now and then, yet an under-current of thought was going on in my mind, which gave at last a result, whereof it is not too much to say that I felt at once the importance. An electric circuit seemed to close; and a spark flashed forth, the herald (as I foresaw, immediately) of many long years to come of definitely directed thought and work, by myself if spared, and at all events on the part of others, if I should even be allowed to live long enough distinctly to communicate the discovery.

Hamilton adds: “Nor could I resist the impulse — unphilosophical as it may have been — to cut with a knife on a stone of Brougham Bridge, as we passed it, the fundamental formula with the symbols, i, j, k; namely,

i² = j² = k² = ijk = -1

which contains the Solution of the Problem, but of course, as an inscription, has long since mouldered away.” The bridge now bears a permanent plaque marking Hamilton’s achievement (below), and mathematicians undertake an annual walk from Dunsink Observatory to commemorate it.

In the Minnesota Multiphasic Personality Inventory, the subject is asked whether he agrees with a series of statements, such as “There seems to be a lump in my throat much of the time,” “I am not afraid to handle money,” “I feel uneasy indoors,” and “My sleep is fitful and disturbed.” His responses give insights into his personality and psychopathology.

In 2006, poet Katie Degentesh fed these statements into Google and combined the results into a series of poems, which she published as The Anger Scale. Here’s an excerpt from “As a youngster I was suspended from school one or more times for cutting up”:

Everyone knows about Dallas
and its acts of terrifying gorgeousness

a chef in a tall hat piping meringue
discussing the “brain drain”

dropped a slab of concrete on his left foot
before being lured to the guitar

doesn’t recall details of cutting up friend
to create fake masterpiece

Poets Craig Dworkin and Kenneth Goldsmith call this “a ‘pataphysical nosography, evaluating and diagnosing the mental stability of the Internet itself.” But how do we evaluate the results?

Pick any three points on a line and use each pair of them to define a semicircle, as shown.

Now draw a perpendicular between the two smaller semicircles.

Circles c₁ and c₂ will always have the same area.

Considerable amusement was excited, a few years ago, by the announcement that a society for mutual autopsy had been formed among the savants of Paris, with a view to advancing knowledge of the structure and physiology of the brain by a correlation of intellectual characteristics with post mortem appearances. The whole thing was generally regarded as a scientific joke of more than ordinary magnitude. But the society appears to have been a genuine fact, and one of its members, M. Asseline, having recently deceased, his brain was carefully examined by his surviving associates, who made a full report of the result to the Anthropological Society of Paris. The following account of the matter is found in Nature, Aug. 14, 1879, p. 377:

‘M. Asseline died in 1878, at the age of 49. He was a republican and a materialist; was possessed of enormous capacity for work, great faculty of mental assimilation, and an extraordinarily retentive memory; and had a gentle, benevolent disposition, keen susceptibilities, refined taste and subtle wit. As a writer he had always displayed great learning, unusual force of style and elegance of diction, and in his intercourse with others he had been unassuming, sensitive and even timid. Yet the autopsy showed such coarseness and thickness of the convolutions that M. Broca pronounced them to be characteristic of an inferior brain. The fossa or depressions, regarded by Gratiolet as a simian character, and as a sign of cerebral inferiority which are often found in women, and in some men of undoubted intellectual inferiority, were very much marked, especially on the left parietooccipital. But the cranial bones were at some points so thin as to be translucent; the cerebral depressions were deeply marked, the frontal suture was not wholly ossified, a decided degree of asymmetry was manifested in the greater prominence of the right frontal, while, moreover, the brain weighed 1,468 grams, i.e., about 60 grains above the average given by M. Broca for M. Asseline’s age. The apparent contradictions between the weight of the brain and the marked character of the parieto-occipital depressions, attracted much attention, and the members of the Société d’Anthropologie have been earnestly invited by M. Hovelacque, in furtherance of science, to join the Société d’Autopsie, to which anthropology is already indebted for many highly important observations. This society is forming a collection of photographs of its members, which are taken in accordance with certain fixed rules.’

— Chicago Medical Journal and Examiner, quoted in New Orleans Medical and Surgical Journal, January 1880

In a 2002 article in Nature, Australian mathematician Burkard Polster concluded that most of us are doing a pretty good job lacing our shoes: “No matter whether you prefer to lace them straight or criss-crossed, you come close to maximizing the total horizontal tension when you pull on the two ends of one of your shoelaces.”

When it comes to tying them, though, we don’t do so well. “A very large number of people, possibly even the majority, do tie their shoe laces much worse than the rest,” Polster wrote in his 2006 book-length followup, The Shoelace Book. Most of us tie a shoe by placing one half-granny knot on top of another, but this can produce either a very unstable granny knot (left) or a very stable reef knot (right), depending on whether the two half-knots have the same or opposite orientation. (It’s not essential that the second half-granny is typically tied with loops; these are omitted in the diagrams.)

“Hundreds of years of trial and error have led to the strongest way of lacing our shoes,” Polster wrote in Nature, “but unfortunately the same cannot be said about the way in which most of us tie our shoelaces — with a granny knot.”

(Burkard Polster, “What Is the Best Way to Lace Your Shoes?” Nature 420:6915 [Dec. 5, 2002], 476.)

When I was a child, it was believed that animals became extinct because they were too specialized. My father used to tell us about the saber-tooth tiger’s teeth — how they got too big and the tiger couldn’t eat because he couldn’t take game anymore. And I remember my father saying, with my brother sitting there, ‘I wonder what it will be with the human beings that will be so overspecialized that they’ll kill themselves off?’

My father never found out that my brother was working on the bomb.

— Richard Feynman’s sister Joan, quoted in Christopher Sykes, No Ordinary Genius, 1994

I am determined & feel sure, that the scenery of England is ten times more beautiful than any we have seen.– What reasonable person can wish for great ill proportioned mountains, two & three miles high? No, no; give me the Brythen or some such compact little hill.– And then as for your boundless plains & impenetrable forests, who would compare them with the green fields & oak woods of England?– People are pleased to talk of the ever smiling sky of the Tropics: must not this be precious nonsense? Who admires a lady’s face who is always smiling? England is not one of your insipid beauties; she can cry, & frown, & smile, all by turns.– In short I am convinced it is a most ridiculous thing to go round the world, when by staying quietly, the world will go round with you.

— Charles Darwin, letter to his sister, July 18, 1836. He was on board the Beagle, bound for Ascencion. He had written the previous December, “How glad I shall be, when I can say, like that good old Quarter Master, who entering the Channel, on a gloomy November morning, exclaimed, ‘Ah here there are none of those d—-d blue skys’.”

An energetic boy got a piggy bank for his birthday. He decided that from then on he will number every bill he gets from his grandparents (1, 2, …) and put it all in his bank. During the first half year he got 2 bills, but at the end of this period he pulled out 1 bill (chosen at random). In the next 1/4 year he got 2 more bills, but at the end of this period he pulled 1 bill chosen at random from the 3 bills in his bank. In the next 1/8 year he repeated the same routine etc. (each period is half the length of the previous period). What is the probability that any of the bills he got during this year will remain in his bank after a full year of the above activity? Paradoxically the probability is 0, even though it is clear that he only spent half of his money. Can we offer the boy good financial advice without making him cut his expenses?

— Talma Leviatan, “On the Use of Paradoxes in the Teaching of Probability,” Proceedings of ICOTS 6, 2002

Along with art and love, life is one of those bedeviling concepts that we really ought to have a definition for but don’t. Philosophers tend to regard the question as too scientific, and scientists as too philosophical. Linus Pauling observed that it’s easier to study the subject than to define it, and, J.B.S. Haldane noted, “no definition will cover its infinite and self-contradictory variety.”

Classical definitions of life typically refer to structural features, growth, reproduction, metabolism, motion against force, response to stimuli, evolvability, and information content and transfer. But definitions built on these elements are prone to exceptions. Fire grows, moves, metabolizes, reproduces, and responds to stimuli, but is “nonliving.” So are free-market economies and the Internet, which evolve, store representations of themselves, and behave “purposefully.” I am nonreproducing but, I hope, still alive.

If we we look around us, it’s hard to find a property that’s unique to life, and even if we could, our observations are limited to Earth’s biosphere, a tiny, tenuous environment like a film of water on a basketball. But if we expand our list to include abstract properties such as resistance to entropy, then we risk including alien phenomena that we might not regard intuitively as living.

Perhaps the answer is more poetic. “As I see it, the great and distinguishing feature of living things … is that they have needs — continual, and, incidentally, complex needs,” wrote botanist Donald C. Peattie in 1935. “I cannot conceive how even so organized a dead system as a crystal can be said to need anything. But a living creature, even when it sinks into that half-death of hibernation, even the seed in the bottom of the driest Mongolian marsh, awaiting rain through two thousand years, still has needs while there is life in it.”

In studying the parasitic protozoan Plasmodium ovale in 1954, English parasitologist William Cooper volunteered to receive the bites of about a thousand mosquitos, and nine days later underwent a laparotomy in which a piece of his liver was removed. On recovering, he stained the sections himself, located the malaria parasite stages in his own tissue, and painted these in watercolors to accompany the resulting article.

His coauthor, University of London protozoologist Cyril Garnham, wrote that Cooper “attained everlasting fame by this episode.”

(P.C.C. Garnham et al., “The Pre-Erythrocytic Stage of Plasmodium Ovale,” Transactions of the Royal Society of Tropical Medicine and Hygiene 49:2 [March 1955], 158-167) (Thanks, Andrew.)