Do you believe in ghosts? Then ghosts are in your ontology, along with tables and chairs and songs and vacations, and snow, and all the rest. It has proved more than convenient to extend the term "ontology" beyond this primary meaning and use it for the set of "things" that an animal can recognize and behave appropriately with regard to (whether or not animals can properly be said to have beliefs) and — more recently — the set of "things" a computer program has to be able to deal with to do its job (whether or not it can properly be said to have beliefs). Vacations are not in the ontology of a polar bear, but snow is, and so are seals. Snow is probably not in the ontology of a manatee, but outboard-motor propellers may well be, along with seaweed and fish and other manatees. The GPS system in your car handles one-way streets, left and right turns, speed limits, and the current velocity of your car (if it isn't zero, it may not let you put in a new target address), but its ontology also includes a number of satellites, as well as signals to and from those satellites, which it doesn't bother you with, but needs if it is to do its job.