“To a man with a hammer, everything looks like a nail.”

Alternate version: If you don't have a hammer, you don't want anything to look like a nail.
Of Paradise and Power, p. 26
According to Kagan, this is a variation of the proverb "When you have a hammer, all problems start to look like nails." (p. 25 of the same book)

The Psychology of Science: A Reconnaissance (1966), Ch. 2, p. 15; although some similar statements to describe fundamental errors in human perception have been attributed to others, his expression, or slight paraphrases of it, is one of the earliest yet found to be documented in published writings, and remains among the most popular.
1940s-1960s

Egwene al'Vere
(15 October 1991)

"I am Goya"; translated by Stanley Kunitz, p. 3.
Antiworlds, and the Fifth Ace

Source: Immortal Beloved

At an interview with Stephen Colbert at Montclair Kimberley Academy on January 29th, 2010.
2010s
Variant: If you start wielding a hammer, then all your problems look like nails. And maybe they’re not. Maybe it's more subtle than that. And so your toolkit has to be able to morph into what is necessary for what it is that you confront at that moment.

Cave on his interest in Eastern and nontheistic spirituality
God and religion