An object is a thing that exists as physical matter. It has mass, takes up space, and can be observed. A rock, a cup, a chair, a person’s body, a planet — all objects.

Objects have two properties that non-physical things lack: they are massive (they have weight and resist being moved) and they are notable (they can be seen, touched, or otherwise sensed). A thought has neither property. A number has neither. An object has both.

Objects occupy locations. A cup is on the table. A book is in the bag. A ship is at port. At any given moment, an object is somewhere — inside a container, on a surface, in an area. Objects move between locations: you pick the cup up, carry it to the kitchen, set it down. The cup’s identity persists through the move. Location changes; the object stays the same object.

Objects can be moved or be stationary. In TAB’s model hierarchy, a mobile is an object that moves under its own power or by being carried. A chair is an object but not a mobile — it stays where you put it unless someone moves it. A person is a mobile — they walk from room to room on their own.

Objects can be inside containers but are not themselves containers unless they have an inside. A ball is an object but not a container — there is no inside to put things in. A box is both an object and a container — it has mass and takes up space (object), and it has an inside where you can put things (container). A bag, a jar, a room — all objects that are also containers.

The distinction between object and thing matters because some things are not objects. A rule is a thing but not an object. A promise is a thing but not an object. An institution is a thing but not an object. The word “object” picks out the physical subset of things — the ones you can drop on your foot.