“To live among friends is the primary essential of happiness.”
Lord Kelvin’s Replies to Addresses given on the Celebration of the Jubilee of his Professorship (June 15-17, 1896). Quoted in Lord Kelvin, Professor of Natural Philosophy in the University of Glasgow 1846-1899 (1899) by George F. Fitzgerald http://historical.library.cornell.edu/cgi-bin/cul.math/docviewer?did=03620002
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William Thomson 18
British physicist and engineer 1824–1907Related quotes

“Happiness is not essential to the artist; happiness never creates anything but memories.”
Oscar Wilde ([1916] 1997) ch. 21, p. 254.

Un Art de Vivre (The Art of Living) (1939), The Art of Happiness

“Employment is Nature's physician, and is essential to human happiness.”
Latter day attributions
Source: Day's Collacon: an Encyclopaedia of Prose Quotations, (1884), p. 223.

Source: Sexual Personae: Art and Decadence from Nefertiti to Emily Dickinson (1990), p. 508

For My Legionaries: The Iron Guard (1936), Jewish Problem

“All things are in common among friends.”
Diogenes, 6.
The Lives and Opinions of Eminent Philosophers (c. 200 A.D.), Book 6: The Cynics