Wittgenstein's ontology consists of objects and states of affairs, both of which exist in logical space. Let us first consider what he says about objects in 2.01–20141. Wittgenstein never gives us an example of what he means by "object," causing a great deal of debate among interpreters.

He introduces objects at 2.01 as the constituents of states of affairs, and elaborates at 2.011 that the possibility of constituting states of affairs is the essence of an object. Philosophers talk about the "essence" of a thing as the properties which make the thing what it is, without which it would be something different. Aristotle defined humans as rational animals: we can lose a leg and still be human, but we can't be human if we aren't rational. Wittgenstein is suggesting that the essence of any object is not certain properties that hold of it, but the possibility of certain kinds of properties holding of it.

Wittgenstein illustrates this point at 2.0131: "A speck in the visual field, though it need not be red, must have some color: it is, so to speak, surrounded by color-space. Notes must have some pitch, objects of the sense of touch some degree of hardness, and so on." An object need not exist in any particular state of affairs, but it must exist in some state of affairs. In the same way that notes must have some pitch, and that pitch exists in what we might call "sound-space," objects must exist in some states of affairs, and those states of affairs exist in logical space.

We might visualize logical space as a grid of light bulbs. Each light bulb represents a possible state of affairs. Behind this grid we can imagine objects as independent power sources, each with wires (different kinds of wires for different kinds of objects) coming out of them, connecting with all light bulbs that have sockets for that particular kind of wire. Any given light bulb will have a number of different wires plugging into it, just as any state of affairs is a combination of a number of different objects. Depending on how these different wires plug into the bulb, they will either conflict, and prevent any power coming to the bulb, or they will combine to turn the light bulb on. Lit light bulbs represent states of affairs that are the case, and unlit light bulbs represent states of affairs that are not the case. The world is the totality of lit light bulbs, whereas unlit light bulbs represent possibilities that are not the case.

This metaphor helps explain a number of key points. First, states of affairs are mutually independent: any given light bulb can be lit or unlit without affecting any of the other light bulbs. Second, the essence of an object is its possibility of existing in certain states of affairs: the significance of the power source lies in which light bulbs it plugs into, and how it does so. Third, the world is "the totality of facts, not of things" (1.1): the world is the totality of lit light bulbs, not of power sources.

Wittgenstein's ontology of states of affairs in logical space effectively limits what there is to what is the case. We cannot say that certain things exist or don't exist: we can only state facts about the world that relate these things to other things and properties. The only way to deal with the question of whether or not unicorns exist is to list all the facts that are the case about unicorns. When we find that there has never been a reliable sighting of a unicorn, and that they have only been encountered in imaginative fiction, we might draw certain conclusions about whether there are any flesh and blood unicorns. To exclude unicorns from our ontology altogether does not even make sense, since it is an ontology made up of facts, and not things. Unicorns can be said not to exist only to the extent that there are no true facts that assert something about unicorns as flesh and blood creatures.

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