 The world seems to consist of stuff and things. Some of the stuff composes things—that wood constitutes this ship. Some things endure for a long time—the 2nd century Pantheon building in Rome—others are transient—a sandcastle on the beach. People care, to varying degrees, what kind of thing is being considered, and whether the one they are discussing is the same one. The Pantheon in Rome today is the same one that was there in AD 150; the sandcastle on the beach today, though it is in the same location and looks the same, is not the one the tide washed away last night. Why do we care about objects and not just the stuff? Because objects are stuff with extra properties, properties that often persist and are often useful to know about even if the stuff changes.