objects in space do not have mental properties. They just don't.
In order to assign the objects to categories (one of the perennial tasks of philosophy), we have to choose something which is more or less common to the objects considered. This becomes the mental property, allowing us to talk of the objects as a class and not just of the unique, individual piece in front of our eyes.
Sometimes this is not so trivial: what mental property allows us to distingish a pear tree from a cherry tree in the middle of winter?