"From each according to his ability, to each according to his needs"
(Karl Marx, "Critique of the Gotha Program", cited by Howard Zinn in "Passionate Declarations", chapter "Economic justice: the American class system").
This is one of the three rules of Emmaüs, recalled by Abbé Pierre in "Mémoire d'un croyant" (chapter "Emmaüs"):
"Nous partageons: le plus costaud, qui apporte beaucoup à la communauté, n'a pas plus que le petit vieux improductif."
For me this is the fundamental concept underlying the left policies, and why I am a leftist.