You will have received my telegram. It all happened terribly quickly. After the best prospects there was a sudden collapse of strength this morning, then he  simply fell asleep. In two minutes this genius had ceased to think, and exactly at the time when the physicians encouraged us to hope for the best. What this man was for us as regards theory, and at all decisive moments also with regard to practical matters, can be understood only by one who was constantly with him. His wide horizons will disappear with him from the scene for many years. These are matters we are not yet equal to. The movement will proceed along its course but it will miss his calm, timely and considered intervention, which hitherto saved it from many a wearisome erroneous path. Further particulars soon. It is now 12 o'clock at night and I have had to write letters and attend to all kinds of things the whole afternoon and evening.
Letters: Karl Marx and Frederick Engels, Selected Correspondence 1883