Sight
Holmes can remember a time before, when he could look at an article of clothing and see it as a utensil for warmth, and not a vehicle for information. He can remember when he was too young to defend himself, and when his bread money was stolen by other, larger children.
There was a certain agitation that accompanied his innocence-- because it was an innocence-- that delicate frustration of wanting to have and being denied the very foundations of knowledge-- and he, a younger man, saw it as an unfortunate life companion and slew it as efficiently as he could.
Holmes couldn't regret it now, perhaps because he knew that it was only his subsequent knowledge that allowed him to recognize his past folly, but there were days that he spent considering the futility of intellect and the inevitability of entropy.
Those were the days he contented himself with his cocaine bottle, the days he would lie in a stupor until some higher power relieved him of his ennui, brought him back into a world in which his knowledge was useful, enjoyable. A world he understood, and appreciated more because of his understanding.
Watson was usually that higher power. Watson, the one who could pull his pillows out from underneath him and his ounce of shag away from him, the one who could force him in front of a wash basin to shave away the accumulated symptoms of apathy. Watson, the eternal puzzle: not in fact, but in potential.
Holmes can remember a time before he met Watson, a time when he could look at a man and see him as nothing more than a receptacle for ideas or facts. He doesn't miss it.