From the fine folks at Oregon State University:
Pears picked when slightly immature will ripen with better quality than pears that are over mature when picked. To tell if a pear is mature, a general rule of thumb is that, while still on the tree, most mature, ready to ripen pears will usually detach when "tilted" to a horizontal position from their usual vertical hanging position. Bosc pears always are difficult to separate from the spur.
"Unlike apples, which are ready to eat from the day they are picked, pears must go through a series of changes before they can deliver their full splendor," explained Sugar. "Pears do not ripen on the tree to our liking. If allowed to tree-ripen, pears typically ripen from the inside out, so that the center is mushy by the time the outside flesh is ready."