“Those who cannot love do not understand it”

Book XXIII, ¶ 7,as rendered in the epigraph to Ch. 3 of The Lathe of Heaven (1971) by Ursula K. Le Guin, based upon the 1891 translation by James Legge, Le Guin was subsequently informed that this was a very poor translation, as there were no lathes in China in the time of Zhuangzi. The full passage as translated by Legge reads:
He whose mind is thus grandly fixed emits a Heavenly light. In him who emits this heavenly light men see the (True) man. When a man has cultivated himself (up to this point), thenceforth he remains constant in himself. When he is thus constant in himself, (what is merely) the human element will leave him, but Heaven will help him. Those whom their human element has left we call the people of Heaven. Those whom Heaven helps we call the Sons of Heaven. Those who would by learning attain to this seek for what they cannot learn. Those who would by effort attain to this, attempt what effort can never effect. Those who aim by reasoning to reach it reason where reasoning has no place. To know to stop where they cannot arrive by means of knowledge is the highest attainment. Those who cannot do this will be destroyed on the lathe of Heaven.

The Rebel (1951)

This and many similar quotes with the same general meaning are misattributed to Aristotle as a result of Twitter attribution decay. The original source of the quote remains anonymous. The oldest reference resides in the works of George Bernard Shaw, Man and Superman (1903): "Maxims for Revolutionists", where he claims that “He who can, does. He who cannot, teaches.”. However, the related quote, "Those who can, do. Those who understand, teach" likely originates from Lee Shulman in his explanation of Aristotlean views on professional mastery: Source: Shulman, L. S. (1986). Those who understand: Knowledge growth in teaching. Educational Researcher, 15(2), 4 - 14. Stable URL: http://www.jstor.org/stable/1175860
Misattributed
Variant: Those who can, do, those who cannot, teach.

"The Mirror of Eternal Salvation", as quoted in Ruysbroeck (1915) by Evelyn Underhill, p. 52

"The Case for the Ephemeral"
All Things Considered (1908)
Context: I cannot understand the people who take literature seriously; but I can love them, and I do. Out of my love I warn them to keep clear of this book. It is a collection of crude and shapeless papers upon current or rather flying subjects; and they must be published pretty much as they stand. They were written, as a rule, at the last moment; they were handed in the moment before it was too late, and I do not think that our commonwealth would have been shaken to its foundations if they had been handed in the moment after. They must go out now, with all their imperfections on their head, or rather on mine; for their vices are too vital to be improved with a blue pencil, or with anything I can think of, except dynamite.
Their chief vice is that so many of them are very serious; because I had no time to make them flippant. It is so easy to be solemn; it is so hard to be frivolous.

Source: The Roominghouse Madrigals: Early Selected Poems, 1946-1966