Homo unius libri ("(a) man of one book") is a Latin phrase attributed to Thomas Aquinas in a literary tradition going back to at least the 17th century, bishop Jeremy Taylor (1613–1667) being the earliest known writer in English to have done so. Saint Thomas Aquinas is reputed to have employed the phrase "hominem unius libri timeo" (meaning "I fear the man of a single book").
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There are other attributions, and variants of the phrase. Variants include cave for timeo, and virum or lectorem for hominem. The Concise Dictionary of Foreign Quotations (London 1998), attributes the quote to Augustine of Hippo. Other attributions named Pliny the Younger, Seneca, Quintilian or Augustine, but the existence of the phrase cannot be substantiated as predating the early modern period.
The phrase was in origin a dismissal of eclecticism, i.e. the "fear" is of the formidable intellectual opponent who has dedicated himself to and become a master in a single chosen discipline; however, the phrase today most often refers to the interpretation of expressing "fear" of the opinions of the illiterate man who has "only read a single book".
Mastery of a single topic
The literary critic Clarence Brown described the phrase in his introduction to a novel by Yuri Olesha:
"[Aquinas's] words are generally quoted today in disparagement of the man whose mental horizons are limited to one book. Aquinas, however, meant that a man who has thoroughly mastered one good book can be dangerous as an opponent. The Greek poet Archilochus meant something like this when he said that the fox knows many things but the hedgehog knows one big thing."The poet Robert Southey recalled the tradition in which the quotation became embedded:
"When St Thomas Aquinas was asked in what manner a man might best become learned, he answered, 'By reading one book'; 'meaning,' says Bishop Taylor, 'that an understanding entertained with several objects is intent upon neither, and profits not. The homo unius libri is indeed proverbially formidable to all conversational figurantes. Like your sharp-shooter, he knows his piece perfectly, and is sure of his shot." — Robert Southey, The Doctor, p. 164.By way of comparison, Southey quotes Lope de Vega's Isidro de Madrid expressing a similar sentiment,
For a noteworthy student is he, The man of a single book. For when they were not filled up With so many extraneous books,The writer and naturalist Charles Kingsley, following the tradition laid down by Gilbert White in The Natural History and Antiquities of Selborne (1789), also invoked the proverb in favour of knowing completely one small area. "A lesson is never learnt till it is learnt over many times, and a spot is best understood by staying in it and mastering it. In natural history the old scholar's saw Cave hominem unius libri may be paraphrased by, 'He is a thoroughly good naturalist who knows one parish thoroughly.'"
John Wesley invoked the phrase in this sense and declared himself to be a "homo unius libri", the "one book" being the Bible. Like Methodists in the strict Wesleyan tradition of depending upon the One Book, many seventeenth century and modern radical Protestants pride themselves on being homines unius libri. The poet William Collins in his deranged condition, met by chance his old friend Dr. Samuel Johnson, who found Collins carrying a New Testament. "I have but one book," said Collins, "but it is the best".
Narrow learning
Edward Everett applied the remark "not only to the man of one book, but also to the man of one idea, in whom the sense of proportion is lacking, and who sees only that for which he looks."
Joseph Needham, in the general conclusions to his Science and Civilisation in China series, observed of the saying, "It could mean that this man has only read one book, has only written one book, does not possess more than one book, or puts his faith in one book only. The fear that is felt may be on behalf of the man himself. Having read so little he is quite at the mercy of his one book!"
Limited success as an author
In Samuel Butler's The Way of All Flesh (1903), a publisher uses the phrase to describe the novel's protagonist. Butler also records that his own publisher, Trübner, applied the phrase to him to express doubts in his literary prospect, the "one book" of Butler's in this case being Erewhon.