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• In scientific thought we adopt the simplest theory which will explain all the facts under consideration and enable us to predict new facts of the same kind. The catch in this criterion lies in the world "simplest." It is really an aesthetic canon such as we find implicit in our criticisms of poetry or painting. The layman finds such a law as dx/dt = K(d²x/dy²) much less simple than "it oozes," of which it is the mathematical statement. The physicist reverses this judgment, and his statement is certainly the more fruitful of the two, so far as prediction is concerned. It is, however, a statement about something very unfamiliar to the plainman, namely, the rate of change of a rate of change.
  • Possible Worlds and Other Papers (1927), p. 227

• I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.
  • Possible Worlds and Other Papers (1927), p. 286
  • Similar remarks that seem derived from this have in recent years been attributed to Arthur Stanley Eddington, as well as to Haldane, but without citations of an original source:

The universe is not only stranger than we imagine, it is stranger than we can imagine.
The world is not only stranger than we imagine, it is stranger than we can imagine.
Not only is the universe stranger than we imagine, it is stranger than we can imagine.

• To the biologist the problem of socialism appears largely as a problem of size. The extreme socialists desire to run every nation as a single business concern. I do not suppose that Henry Ford would find much difficulty in running Andorra or Luxembourg on a socialistic basis. He has already more men on his pay-roll than their population. It is conceivable that a syndicate of Fords, if we could find them, would make Belgium Ltd. or Denmark Inc. pay their way. But while nationalization of certain industries is an obvious possibility in the largest of states, I find it no easier to picture a completely socialized British Empire or United States than an elephant turning somersaults or a hippopotamus jumping a hedge.
  • "On Being the Right Size" in Possible Worlds (1928)

• My practice as a scientist is atheistic. That is to say, when I set up an experiment I assume that no god, angel, or devil is going to interfere with its course; and this assumption has been justified by such success as I have achieved in my professional career. I should therefore be intellectually dishonest if I were not also atheistic in the affairs of the world.
  • Fact and Faith (1934) Preface

• It seems to me immensely unlikely that mind is a mere by-product of matter. For if my mental processes are determined wholly by the motions of atoms in my brain I have no reason to suppose that my beliefs are true. They may be sound chemically, but that does not make them sound logically. And hence I have no reason for supposing my brain to be composed of atoms.
  • "When I am Dead" in Possible Worlds (1927)

• I had it for about fifteen years until I read Lenin and other writers, who showed me what was wrong with our society and how to cure it... Since then I have needed no magnesia.
  • On being cured of his gastritis, as quoted in TIME magazine (24 June 1940)

• An inordinate fondness for beetles.
  • A possibly apocyrphal reply to theologians who inquired if there was anything that could be concluded about the Creator from the study of creation; as described in "Homage to Santa Rosalia, or why are there so many kinds of animals" by G. Evelyn Hutchinson in American Naturalist (May-June 1959); This alludes to the fact that there are more types of beetles than any other form of insect, and more insects than any other kind of animal.
  • Unsourced variants:
    The Creator, if He exists, has "an inordinate fondness for beetles".
    If one could conclude as to the nature of the Creator from a study of creation, it would appear that God has an inordinate fondness for stars and beetles.
    The Creator, if He exists, has a special preference for beetles, and so we might be more likely to meet them than any other type of animal on a planet that would support life.

• I suppose the process of acceptance will pass through the usual four stages:
  (i) this is worthless nonsense;
  (ii) this is an interesting, but perverse, point of view;
  (iii) this is true, but quite unimportant;
  (iv) I always said so.
  • Journal of Genetics Vol. 58, page 464 (1963).

• An ounce of algebra is worth of a ton of verbal argument.
  • As quoted in his obituary by Maynard Smith in Nature 206 (1965), p. 239

• No, but I would to save two brothers or eight cousins.
  • Reply when asked if he would give his life to save a drowning brother, as quoted in Mathematical Models of Social Evolution : A Guide for the Perplexed (2007) by Richard McElreath and Robert Boyd, p. 82; as you share on average half your alleles with a brother and one-eighth with a cousin, Haldane was giving the number of relatives one would have to save to "break even".

Daedalus or Science and the Future (1923)

An address "Daedalus or Science and the Future" (4 February 1923)

• There is no great invention, from fire to flying, which has not been hailed as an insult to some god.

• The conservative has but little to fear from the man whose reason is the servant of his passions, but let him beware of him in whom reason has become the greatest and most terrible of the passions. These are the wreckers of outworn empires and civilisations, doubters, disintegrators, deicides.

• Science is as yet in its infancy, and we can foretell little of the future save that the thing that has not been is the thing that shall be; that no beliefs, no values, no institutions are safe. So far from being an isolated phenomenon the late war is only an example of the disruptive result that we may constantly expect from the progress of science. The future will be no primrose path. It will have its own problems. Some will be the secular problems of the past, giant flowers of evil blossoming at last to their own destruction. Others will be wholly new. Whether in the end man will survive his ascensions of power we cannot tell. But the problem is no new one. It is the old paradox of freedom re-enacted with mankind for actor and the earth for stage.

• The time has gone by when a Huxley could believe that while science might indeed remould traditional mythology, traditional morals were impregnable and sacrosanct to it. We must learn not to take traditional morals too seriously. And it is just because even the least dogmatic of religions tends to associate itself with some kind of unalterable moral tradition, that there can be no truce between science and religion.
  There does not seem to be any particular reason why a religion should not arise with an ethic as fluid as Hindu mythology, but it has not yet arisen. Christianity has probably the most flexible morals of any religion, because Jesus left no code of law behind him like Moses or Muhammad, and his moral precepts are so different from those of ordinary life that no society has ever made any serious attempt to carry them out, such as was possible in the case of Israel and Islam. But every Christian church has tried to impose a code of morals of some kind for which it has claimed divine sanction. As these codes have always been opposed to those of the gospels a loophole has been left for moral progress such as hardly exists in other religions. This is no doubt an argument for Christianity as against other religions, but not as against none at all, or as against a religion which will frankly admit that its mythology and morals are provisional. That is the only sort of religion that would satisfy the scientific mind, and it is very doubtful whether it could properly be called a religion at all.

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• I will give up my belief in evolution if someone finds a fossil rabbit in the Precambrian.