Joseph Stalin Collected Works, volume one to twelve:
#as requested by materialismo-dialectico
"Theory is the experience of the working-class movement in all countries taken in its general aspect. Of course, theory becomes purposeless if it is not connected with revolutionary practice, just as practice gropes in the dark if its path is not illumined by revolutionary theory. But theory can become a tremendous force in the working-class movement if it is built up in indissoluble connection with revolutionary practice; for theory, and theory alone, can give the movement confidence, the power of orientation, and an understanding of the inner relation of surrounding events; for it, and it alone, can help practice to realise not only how and in which direction classes are moving at the present time, but also how and in which direction they will move in the near future."