In “The Adventure of the Empty House,” an old book collector visits John Watson’s house, “his precious volumes, a dozen of them at least, wedged under his right arm.” He says, “Maybe you collect yourself, sir; here’s British Birds, and Catullus, and The Holy War — a bargain every one of them. With five volumes you could just fill that gap on that second shelf.”
Now, this must mean either that two of the titles comprised two volumes apiece or that one comprised three volumes, a point first made by Magistrate S. Tupper Bigelow. But which is it? In the strangely half-specified world that Holmes and Watson inhabit, the fact of the matter seems not to exist.
Philosopher Terence Parsons asks whether Holmes has a mole on his back. Since the stories are silent on this point, it seems that he neither has one nor doesn’t.
See Truth and Fiction.