kinds of things: (a) things which exist necessarily and (b) things which are produced by some external cause. Let me explain. (a) Things which exist necessarily exist by a necessity of their own nature. It’s impossible for them not to exist. Philosophers call such things metaphysically necessary beings. Examples? Many mathematicians think that numbers, sets, and other mathematical entities exist in this way. It’s hotly debated whether such mathematical entities really do exist. But virtually everyone