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Who really said it

Beware the man of one book

Not Thomas Aquinas — a later proverb Anonymous proverb Tied to Aquinas through an anecdote in Jeremy Taylor’s The Great Exemplar (1649)
Disputed Rights unconfirmed

The line does not appear in anything Aquinas wrote, and Wikiquote ↗ lists it as disputed. What can be documented is a second-hand story in Bishop Jeremy Taylor’s The Great Exemplar (1649). In it, Aquinas is asked how a man might best become learned and answers “By reading of one book.” That is advice to focus, not a warning. The “beware” wording is a later proverb built on that story.

Last verified 3 Nov 2026 · how we verify · spot an error?

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Disputed — copy with the correction

“Beware the man of one book” is not found in Thomas Aquinas’s works; it is a later proverb. Behind it is an anecdote in Jeremy Taylor’s The Great Exemplar (1649): Aquinas, asked how a man might best become learned, answered “By reading of one book.” (Quotle.info)

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We couldn’t verify where this wording comes from, so there’s no rights status to give. Safe to present as an unverified or anonymous line — just don’t assert a specific author or source.

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In context

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Provenance

The anecdote behind the attribution

Work
The Great Exemplar of Sanctity and Holy Life
Author
Jeremy Taylor (1613–1667), Anglican bishop
Location
Part II, Section XII, Discourse XII (“Of prayer”), §16
Edition shown
Whole Works, vol. II, ed. Reginald Heber, rev. Charles Page Eden (London, 1847)

…so is the spirit of man busied in variety, and divided in itself: it abates its fervour, cools into indifferency, and becomes trifling by its dispersion and inadvertency. Aquinas was once asked, with what compendium a man might best become learned? he answered, By reading of one book: meaning, that an understanding entertained with several objects is intent upon neither, and profits not. Taylor, 1649

This is Taylor’s anecdote as printed in the 1847 edition of his works. The same passage appears in the 1649 printing. Taylor gives no source for the story, and in this passage Aquinas recommends one book; nobody is told to beware of anyone. The main text gives his answer only in English, not as the Latin timeo (“I fear”) phrase.

1649 printing (scan)
The Great Exemplar, 1649 printing, at the Internet Archive
Taylor, Whole Works vol. II (1847), Internet Archive

How the trail runs

  • Wikiquote ↗ lists Hominem unius libri timeo (“I fear the man of a single book”) under Disputed on its Aquinas page. Its only citation is Leonard Sweet’s 2012 book, and it gives “Beware the man of one book” as a variant.
  • Jeremy Taylor’s The Great Exemplar (1649) tells the story that Aquinas, asked how to become learned, answered “By reading of one book,” meaning a divided mind “profits not.” (1847 edition ↗)
  • Bartlett’s Familiar Quotations (ninth edition) lists “Beware of a man of one book” as a proverbial phrase on p. 853. It explains it with Robert Southey’s retelling of Taylor’s anecdote from The Doctor. (Project Gutenberg ↗)
  • Wikipedia ↗ notes that the phrase first praised mastery of one discipline, and John Wesley used it of himself and the Bible. Today it usually means the opposite: someone narrow, who has read only one book.

The popular English wording has no identifiable author or first publication, so we assert no rights status for it. The sources excerpted here, Taylor’s The Great Exemplar (1649; 1847 edition) and Bartlett’s Familiar Quotations (ninth edition), were published before 1931 and are public domain in the US.

Rights status is our research, not legal advice — and copyright varies by country and edition. For commercial or published use, confirm with the rightsholder. See terms.

Fact-check

How a piece of advice became a warning

“Beware the man of one book” is usually credited to Thomas Aquinas, often with the Latin Hominem unius libri timeo. The paper trail does not support either.

Thomas Aquinas Not in his works

No passage in Aquinas’s writings has been cited for it. Wikiquote ↗ files it under Disputed, and the Catholic writer Taylor Marshall says ↗ it is not in Aquinas’s works as far as he has read.

Jeremy Taylor, The Great Exemplar (1649) Second-hand story

Taylor reports, with no source, that Aquinas answered “By reading of one book” when asked how to become learned. In this passage the one-book reader is the ideal, not someone to fear. (text ↗)

Bartlett’s Familiar Quotations, via Robert Southey Proverb

Bartlett’s ninth edition prints “Beware of a man of one book” as a proverbial heading and explains it with Southey’s retelling of Taylor. Southey says the homo unius libri is “proverbially formidable.” (Gutenberg ↗)

Aquinas is not known to have written “Beware the man of one book.” What can be documented is Jeremy Taylor’s unsourced 1649 story that Aquinas recommended “reading of one book,” and a later proverb that turned that advice into a warning. See Homo unius libri ↗.

Context

Why the meaning flipped

In Taylor’s story, the “one book” is a mark of focus: an understanding “entertained with several objects is intent upon neither, and profits not.” Southey later called the homo unius libri “proverbially formidable,” meaning a debater who has mastered one text and is hard to beat on it.

The modern quotation runs the other way. It warns against the narrow reader who has read only one book. Wikipedia ↗ notes that this is now the usual sense, which reverses the earlier one.

By reading of one book.

John Wesley’s use

Methodism’s founder John Wesley took up the phrase in the older, approving sense, with the one book being the Bible. In a letter to John Newton of 14 May 1765 he wrote, “in 1730 I began to be homo unius libri, to study (comparatively) no book but the Bible.” (as quoted on Wikipedia ↗)

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Filed under

More provenance-checked quotes on the same theme, each with its reuse status.

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Dig deeper

Trusted places to keep pulling the thread — primary archives, fact-checks, and scholarship. We show how we used each one before you leave.

Thomas Aquinas — Disputeden.wikiquote.org

Lists Hominem unius libri timeo / “Beware the man of one book” under Disputed
Homo unius librien.wikipedia.org

History of the phrase: Taylor, Southey, Wesley, and the reversal of its meaning
Jeremy Taylor, Whole Works vol. II (1847)archive.org

Full text of The Great Exemplar, with the Aquinas “one book” anecdote (Pt II, Sect. XII)
The Great Exemplar (1649 printing)archive.org

Scan of the 1649 printing containing the same anecdote
Bartlett’s Familiar Quotations, 9th ed.gutenberg.org

Lists “Beware of a man of one book” as a proverbial phrase (p. 853)
Did Thomas Aquinas Fear the Man of One Book?taylormarshall.com

A Thomist writer notes the line is not in Aquinas’s works as far as he has read

The name it’s pinned to

About Thomas Aquinas

Thomas Aquinas

c. 1225 – 1274 · Italian Dominican friar, theologian & philosopher

Thomas Aquinas was an Italian Dominican friar and priest, and the leading theologian of medieval Scholasticism. He is best known for the Summa Theologiae. People have long credited him with the Latin Hominem unius libri timeo (“I fear the man of a single book”) and its English form “Beware the man of one book,” but neither has been found in his writings.

For writers & researchers

Cite this

Earliest anecdote linking Aquinas to “one book”

Not found in the works of Thomas Aquinas. The anecdote behind the attribution: Jeremy Taylor, The Great Exemplar of Sanctity and Holy Life (London, 1649), pt. II, sect. XII, disc. XII, §16; reprinted in The Whole Works of the Right Rev. Jeremy Taylor, ed. Reginald Heber, rev. Charles Page Eden, vol. 2 (London: Longman, Brown, Green and Longmans, 1847).

This verification page

Quotle.info. “Who really said ‘Beware the man of one book’?” Last verified 3 Nov 2026. https://quotle.info/who-said/beware-the-man-of-one-book/

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