“And action is the only remedy to indifference, the most insidious danger of all.”

"The Convenient Reverse of Logic in Our Time," commencement address, American University (1984); reprinted in A View from the Stands (1986)

State Capitalism and World Revolution (1950)

Original text: La France est la plus brillante et la plus dangereuse des nations de l'Europe, et la mieux faite pour y devenir tour à tour un objet d'admiration, de haine, de pitié, de terreur, mais jamais d'indifférence.
Variant translation: The French constitute the most brilliant and the most dangerous nation in Europe and the best qualified in turn to become an object of admiration, hatred, pity or terror but never indifference.
Old Regime (1856), p. 245 http://books.google.com/books?id=N50aibeL8BAC&pg=PA254&vq=%22the+most+brilliant+and+the+most+dangerous%22&source=gbs_search_r&cad=1_1
1850s and later

The Westminster Review, vol. 6 (1826), p. 13
Context: This habit of forming opinions, and acting upon them without evidence, is one of the most immoral habits of the mind.... As our opinions are the fathers of our actions, to be indifferent about the evidence of our opinions is to be indifferent about the consequences of our actions. But the consequences of our actions are the good and evil of our fellow-creatures. The habit of the neglect of evidence, therefore, is the habit of disregarding the good and evil of our fellow-creatures.

Remark (6 November 1605) as quoted in The Dictionary of National Biography Vol. 6 (1917); He is here invoking a version of a famous statement of Hippocrates, also translated as "Extreme remedies are very appropriate for extreme diseases."

"Indifference: Far And Away The Most Dangerous Human Capacity", 13.7: Cosmos & Culture (28 April 2011)
Context: While I agree… that indifference is indeed our most dangerous capacity, I actually do believe that it's on the wane.
When I scroll back to my 1950's Connecticut girlhood and recall how clueless everyone was about just about everything, how we mindlessly parroted concepts like "Better Dead Than Red" and the "Domino Theory," how my friends were all lily-white and Koreans were gooks and I would have had no idea where to find Nigeria on a map – when I go back there and then think about Adam's students and my students and my kids and what they've come to understand and care about, it gets a whole lot better.

The World Crisis, 1915 : Chapter I (The Deadlock in the West), Churchill, Butterworth (1923), pp. 22-23.
Early career years (1898–1929)
Context: Mechanical not less than strategic conditions had combined to produce at this early period in the war a deadlock both on sea and land. The strongest fleet was paralysed in its offensive by the menace of the mine and the torpedo. The strongest army was arrested in its advance by the machine gun...... The mechanical danger must be overcome by a mechanical remedy..... Something must be discovered which would render ships immune from the torpedo, and make it unnecessary for soldiers to bare their breasts to the machine-gun hail.

Source: I Capture the Castle

Source: UnSouled