Why did Watson consider Holmes' papers his 'great crux'?

Answer 1

"Our chambers were always full of chemicals and of criminal relics, which had a way of wandering into unlikely positions, and of turning up in the butter-dish, or in even less desirable places. But his papers were my great crux. He had a Horror of destroying documents, especially those which were connected with his past cases, and yet it was only once in every year or two that he would muster energy to docket and arrange them, for as I have mentioned somewhere in these incoherent memoirs, the outbursts of passionate energy when he performed the remarkable feats with which his name is associated were followed by reactions of lethargy, during which he would lie about with his violin and his books, hardly moving, save from the sofa to the table. Thus month after month his papers accumulated, until every corner of the room was stacked with bundles of manuscript which were on no account to be burned, and which could not be put away save by their owner." -- John H. Watson, 'The Musgrave Ritual'