“Remember happiness doesn't depend upon who you are or what you have; it depends solely on what you think.”

Illusions : The Adventures of a Reluctant Messiah (1977)

I, 9; translation by W. Hale White (Revised by Amelia Hutchison Stirling)
On the Improvement of the Understanding (1662)

Letter to Arthur Hugh Clough (December 1847/early 1848)
Context: Had Shakespeare and Milton lived in the atmosphere of modern feeling, had they had the multitude of new thoughts and feelings to deal with a modern has, I think it likely the style of each would have been far less curious and exquisite. For in a man style is the saying in the best way what you have to say. The what you have to say depends on your age. In the 17th century it was a smaller harvest than now, and sooner to be reaped; and therefore to its reaper was left time to stow it more finely and curiously. Still more was this the case in the ancient world. The poet's matter being the hitherto experience of the world, and his own, increases with every century.

Variant: It isn't what you have, or who you are, or where you are, or what you are doing that makes you happy or unhappy. It is what you think about.
Source: How to Win Friends and Influence People

Source: The Devil is Dead (1971), Ch. 5

It's always seemed to me that this is precisely the sort of dynamic, positive thinking that we so desperately need today in these trying times of crisis and universal brouhaha.
Introduction to "We Will All Go Together When We Go"
An Evening (Wasted) With Tom Lehrer (1959)

His opinion on the loyalty of Zionists to the United States
Willard Hotel speech (1961)