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Definition 2024

if_all_you_have_is_a_hammer,_everything_looks_like_a_nail

if all you have is a hammer, everything looks like a nail

English

Also often attributed, without citation, to Mark Twain (for example in Jonathan Haidt, The Righteous Mind, page 9).

Proverb

if all you have is a hammer, everything looks like a nail

With limited tools, single-minded people apply them inappropriately or indiscriminately
If a person is familiar with a certain, single subject, or has with them a certain, single instrument, they may have a confirmation bias to believe that it is the answer to/involved in everything.

Etymology

Likely traditional. In this form, perhaps from Abraham Maslow, The Psychology of Science, 1966, page 15 and his earlier book Abraham H. Maslow (1962), Toward a Psychology of Being:

I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail.

Similar concept by Abraham Kaplan, The Conduct of Inquiry: Methodology for Behavioral Science, 1964, page 28:

I call it the law of the instrument, and it may be formulated as follows: Give a small boy a hammer, and he will find that everything he encounters needs pounding.

Labeled "Baruch's Observation" (after Bernard Baruch) in The Complete Murphy's Law: A Definitive Collection (1991) by Arthur Bloch.