“As nothing is more easy than to think, so nothing is more difficult than to think well.”

The Autobiography of a Sexually Emancipated Communist Woman (1926)
Context: Nothing is more difficult than writing an autobiography. What should be emphasized? Just what is of general interest? It is advisable, above all, to write honestly and dispense with any of the conventional introductory protestations of modesty. For if one is called upon to tell about one's life so as to make the events that made it what it became useful to the general public, it can mean only that one must have already wrought something positive in life, accomplished a task that people recognize. Accordingly it is a matter of forgetting that one is writing about oneself, of making an effort to abjure one's ego so as to give an account, as objectively as possible, of one's life in the making and of one's accomplishments.

"Physics and Reality" in the Journal of the Franklin Institute Vol. 221, Issue 3 (March 1936)
Variant translation: "The whole of science is nothing more than a refinement of everyday thinking." As it appears in the "Physics and Reality" section of the book "Out of My Later Years" by Albert Einstein (1950)
1930s

Sparks
The Note-Books of Samuel Butler (1912), Part XIV - Higgledy-Piggledy
Context: Everything matters more than we think it does, and, at the same time, nothing matters so much as we think it does. The merest spark may set all Europe in a blaze, but though all Europe be set in a blaze twenty times over, the world will wag itself right again.

Quote in Vincent's letter, from Arles, Tuesday, 18 September 1888; as cited in Van Gogh : The Self-portraits (1969) by Fritz Erpel, p. 17
Variant translations: The more I think about it, the more I realize there is nothing more artistic than to love others.
As quoted in Mary Engelbreit's Words To Live By (1999) by Mary Engelbreit
I tell you the more I think, the more I feel that there is nothing more truly artistic than to love people.
1880s, 1888
Variant: There is nothing more truly artistic than to love people.

Source: Life and Fate

Source: 1960s, Strength to Love (1963), Ch. 1 : A tough mind and a tender heart
Context: The tough mind is sharp and penetrating, breaking through the crust of legends and myths and sifting the true from the false. The tough-minded individual is astute and discerning. He has a strong austere quality that makes for firmness of purpose and solidness of commitment.
Who doubts that this toughness is one of man's greatest needs? Rarely do we find men who willingly engage in hard, solid thinking. There is an almost universal quest for easy answers and half-baked solutions. Nothing pains some people more than having to think.

Book I, section 42. Translation by Cyrus R. Edmonds (1873), p. 73
De Officiis – On Duties (44 BC)