Derived from the Greek word for being, but a 17th-century coinage for the branch of metaphysics that concerns itself with what exists. Apart from the ontological argument itself there have existed many a priori arguments that the world must contain things of one kind or another: simple things, unextended things, eternal substances, necessary beings, and so on. Such arguments often depend upon some version of the principle of sufficient reason. Kant is the greatest opponent of the view that unaided reason can tell us in detail what kinds of thing must exist, and therefore do exist. In the 20th century, Heidegger was often thought of primarily as an ontologist. Quine’s principle of ontological commitment is that to be is to be the value of a bound variable, a principle not telling us what things exist, but how to determine what things a theory claims to exist. These are the things the variables range over in a properly regimented formal presentation of the theory. Philosophers characteristically charge each other with reifying things improperly, and in the history of philosophy every kind of thing will at one time or another have been thought to be the fictitious result of an ontological mistake.