Homo unius libri

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 " (meaning "I fear the man of a single book").

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".

The literary critic Clarence Brown described the phrase in his introduction to a novel by Yuri Olesha:

The poet Robert Southey recalled the tradition in which the quotation became embedded:

By way of comparison, Southey quotes Lope de Vega's Isidro de Madrid expressing a similar sentiment,

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.'"

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