Popper, Searle and Hildegard: who vouches for what we claim to know

Karl Popper died on Saturday 17 September 1994 at his home in Kenley, on the southern edge of London, at the age of ninety-two. Thirty-one years to the day later, on 17 September 2025, John Searle died in Safety Harbor, Florida, at ninety-three, and the obituaries that followed in October, in the Guardian, The Times and the New York Times, all led with the same thought experiment from 1980 and then, a few paragraphs down, with the same disgrace from 2019. Eight and a half centuries before either of them, on 17 September 1179, Hildegard of Bingen died at Rupertsberg, the convent she had built on a hill above the Rhine against the wishes of the abbot who had hoped to keep her, and her income, at Disibodenberg.

Three deaths and, in Philadelphia on 17 September 1787, one signing. It is tempting to leave them as a list. But they share a question, and it is the question this series exists to ask of a date: when a person says "I know", what is supposed to be standing behind the sentence? Popper answered that nothing is, and that this is fine. Searle answered that a brain is, and that no program can be. Hildegard answered that God was, and had a pope confirm it in writing. The thirty-nine men in the Pennsylvania State House answered that a text is, provided enough people agree to treat it that way, which turns out to be Searle's second answer too.

Popper: knowledge that no one vouches for#

The book that made Popper's reputation among scientists, Logik der Forschung, was printed in Vienna by Springer at the end of 1934 with 1935 on its title page. Its argument is easily stated and was, for its time, rude. The Vienna Circle, whose meetings he was never invited to join, held that a scientific statement is one that observation can verify. Popper replied that no finite run of observations verifies a universal law, that Hume had shown this in 1739, and that the real mark of science is the opposite property: a theory is scientific if it forbids something, so that an observation could refute it. Knowledge, on this account, is a heap of conjectures that have so far survived our attempts to kill them. Nobody vouches for them. The best a theory can hope for is to be still standing tomorrow.

"I think that I have solved a major philosophical problem: the problem of induction," the 1972 collection Objective Knowledge begins, and the sentence has been held against him ever since, because most philosophers of science think the problem was relocated rather than solved. If no amount of confirmation makes a theory more probable, why prefer the well-tested theory when building a bridge? Popper's answer, that we prefer the better-corroborated theory without believing it to be more likely true, has never satisfied anyone who was not already a Popperian. Yet the demarcation criterion outlived the epistemology that produced it. Falsifiability is still what a working scientist reaches for when asked to say why astrology is not science, and the fact that Popper's own account of what falsification requires became more hedged with every edition does not seem to have dented its usefulness.

The political books came from the same instinct. The Open Society and Its Enemies, drafted in exile in New Zealand and published by Routledge in 1945, is remembered for its assault on Plato, Hegel and Marx, and the assault is often unfair, especially to Hegel. Its lasting contribution is a change of question. Political philosophy since Plato had asked who should rule, the wise, the good, the people, the proletariat. Popper argued that this was the wrong question, because any answer to it invites tyranny once the wrong person is installed. The question to ask instead, he wrote, is "how can we so organize political institutions that bad or incompetent rulers can be prevented from doing too much damage?" That is falsificationism applied to government: do not certify the ruler, arrange things so that the ruler can be thrown out. The paradox of tolerance, the most quoted thing he wrote, lives in a footnote to the same chapter and is more cautious than its afterlife on social media suggests. The intolerant are to be suppressed only when they will not meet argument with argument. Popper was at the London School of Economics from 1946 to 1969 and was knighted in 1965.

Searle: the room, the brain and the institution#

Searle was born in Denver on 31 July 1932, went to Oxford as a Rhodes Scholar, wrote his doctorate under J. L. Austin, and taught at Berkeley from 1959 until his emeritus status was revoked on 19 June 2019, after the university found that he had violated its policies on sexual harassment and retaliation in 2016. The finding is part of the record; so is the work.

The work comes in three parts, and only the middle one is famous. Speech Acts (1969) took Austin's observation that saying is a form of doing and made a theory of it: to promise, to warn, to christen, is to perform an act governed by constitutive rules, in the way that castling is governed by the rules of chess. Then, in the third volume of Behavioral and Brain Sciences in 1980, came "Minds, Brains, and Programs", with twenty-seven commentaries attached, and the room. A man who knows no Chinese sits in a room with a rulebook. Chinese characters come in through a slot; he matches them against the rules and pushes other characters out. To those outside, the room speaks Chinese. Inside, nobody understands a word. Searle's conclusion was that a program, being nothing but rules for manipulating symbols, cannot understand, because "syntax is not by itself sufficient for, nor constitutive of, semantics". Understanding is something brains do, in the way that stomachs digest, and it is a biological fact about them that no amount of simulation reproduces.

Forty-six years of replies have not put the argument to rest. The systems reply says that the room as a whole understands even though the man does not; Searle answered that the man could memorise the rulebook and still understand nothing. The robot reply gives the room a body and sensors; Searle answered that this only feeds in more symbols. What has changed is the audience. In 1980 no machine could pass the test the room imagines. In 2026 several can hold a conversation in Chinese, and the Stanford Encyclopedia's entry on the argument now ends by wondering whether, if a system's answers become indistinguishable from a person's, insisting on a "real" understanding that nothing could detect has any content left. This site went over that ground twice after his death, in a piece on the room's legacy and one on what the question still asks. The short version is that the argument is intact and the intuition that powered it is weaker than it was.

The third Searle is the one the day's other anniversaries need. The Construction of Social Reality (1995) asks how there can be objective facts about money, marriages, universities and borders, when none of these would exist if nobody believed in them. His answer is a formula: "X counts as Y in context C". A piece of paper counts as a twenty-dollar bill in the United States; a man counts as president; a signed sheet of parchment counts as the supreme law. What does the counting is collective acceptance, and what the acceptance creates is a status function, a power the object has only because we assign it. A constitution is the purest case there is.

Philadelphia: a text vouched for by agreement#

On the afternoon of 17 September 1787, in the Pennsylvania State House, the delegates who remained at the Federal Convention signed the document they had spent four months arguing over. Three who were present would not: George Mason of Virginia, Elbridge Gerry of Massachusetts and Edmund Randolph, who had introduced the Virginia Plan that started the whole business. Benjamin Franklin, too frail to deliver his own speech, had James Wilson read it for him. It conceded that he did not approve of several parts of the document, doubted he ever would, and urged every delegate to "doubt a little of his own infallibility" and sign. Thirty-nine did. The convention adjourned at four and the members went to the City Tavern for dinner.

Nothing about the parchment made it law. It had to be sent to Congress, then to conventions in the states, and it became binding on 21 June 1788, when New Hampshire's convention made a ninth ratification. That interval is Searle's point exactly. The Constitution is a status function, an X that counts as Y because enough people in the right positions accepted that it did. It is also Popper's point, in a different key. The framers did not ask who should rule. They assumed rulers would be bad and built a machine for limiting the damage: two chambers, a veto, a court, elections at fixed intervals, an amending process hard enough to slow a mob and not so hard as to freeze the text. Whether the machine still does what Madison said it would is a live question, and this site's piece on AI and democratic authorship looks at one version of it. The point for today is narrower. A written constitution is the clearest example we have of knowledge, in this case knowledge of what the law is, that rests on nothing except the continued willingness of people to treat a text as authoritative.

Hildegard: knowledge vouched for by a vision, and a pope#

Hildegard was born around 1098 and was given to the church as a child, enclosed with the anchoress Jutta at Disibodenberg. She said later that she had seen visions since she was three and had told almost nobody. In 1141, at forty-two, she reported a command to write them down, and the book that resulted, Scivias, "Know the Ways", took until 1151 to finish. It runs to twenty-six visions in three parts, each described and then interpreted, and the interpretations amount to a complete theology of creation, redemption and judgement, with a cosmology in which the universe is an egg and the human body a map of it.

What matters for the day's question is how the claim was vouched for. Hildegard was a woman, unordained and, by her own repeated insistence, unlearned in the schools. Her authority to teach could not come from a licence. It came from the visions, and in 1146 she wrote to Bernard of Clairvaux, the most powerful churchman in Europe, to ask whether she should speak or stay silent. He replied, cautiously, that the visions seemed to be from God and that he would not interfere with them. The following winter a delegation carried part of Scivias to the Synod of Trier, where Pope Eugenius III read from it and gave her permission to publish whatever she received. From then on Hildegard corresponded with emperors, preached in public on four tours through the Rhineland, wrote two further books of visions, a natural history, a medical treatise and a body of music, and founded Rupertsberg in 1150 over the objections of her abbot. In the last year of her life the clergy of Mainz placed her convent under interdict for burying an excommunicated nobleman in its grounds; she refused to dig him up, and the interdict was lifted in the spring of 1179, months before she died.

Benedict XVI declared her a Doctor of the Church on 7 October 2012, the fourth woman to receive the title. The philosophers took longer. For most of the twentieth century she belonged to the historians of mysticism and music. What has brought her back into philosophy departments in the last three decades is the concept she called viriditas, greenness, the living force she saw running through plants, bodies and souls alike, and the refusal in her writing to separate the health of a body from the order of the cosmos. Readers now find in it an early attempt to think nature and mind together, and ecological philosophy has claimed her with more enthusiasm than the texts always support. The dry fact remains that her knowledge claim was the most heavily vouched for of anyone on this page, and the one modern readers find hardest to accept.

Bahnsen and the argument that knowledge needs a guarantor#

Greg Bahnsen was born in Auburn, Washington, on 17 September 1948 and died at forty-seven on 11 December 1995, after heart surgery. A minister in the Orthodox Presbyterian Church, he took a doctorate at the University of Southern California in 1978 on self-deception and spent his short career systematising the apologetics of his teacher Cornelius Van Til. The method is called presuppositionalism, and its central move is a transcendental argument: logic, induction and morality all presuppose a mind that guarantees them, so the atheist who reasons at all is borrowing from a God he denies. Bahnsen put the case in a public debate with the atheist Gordon Stein in February 1985 that his followers still circulate on tape.

It belongs here because it is the mirror of Popper. Popper said knowledge can stand with no one vouching for it. Bahnsen said that without a guarantor there is no knowledge at all, only unfounded habit, and that Hume, whom Popper thought had settled the matter, had proved his point rather than Popper's. Most philosophers think the transcendental argument fails, because a precondition of reasoning need not be a person. But the disagreement is real, and it is the same one the day keeps returning to.

Also on the day#

Two meetings open. The Ohio Valley Philosophy of Education Society holds its 2026 annual conference at the Seasons Lodge in Nashville, Indiana, from 17 to 19 September, on the theme of educational loss and hope, beginning today with a pre-conference workshop. The thirteenth International Philosophy of Medicine Roundtable, hosted online by the University of Pittsburgh's Center for Philosophy of Science with the Department of History and Philosophy of Science and the Institute for Bioethics, runs from 14 to 18 September, with sessions today from nine to half past eleven Eastern time. The wider season is in our autumn calendar; the previous instalment of this series is here.