Your question refers to the accident caused by the Monseigneur the Marquis when his carriage ran over the small child on the streets of Paris. The father of the child, overcome with grief, turns to a tall man for comfort who tells him:
"Be a brave man, my Gaspard! It is better for the poor little plaything to die so, than to live. It has died in a moment without pain. Could it have lived an hour as happily?" (Book 2, Chapter 7)
The Monseigneur is impressed with this response, calling him a "philosopher." The tall man, of course, is Monsieur Defarge, and his wife is on the scene too, looking on coldly and knitting furiously. When all the other men were cowering before the Marquis, Madame Defarge did not avert her eyes but rather looked on steadily at the callous Marquis.