Existence is the state of being real. Rocks exist. Numbers exist, in some sense. Unicorns do not. But what does it mean to exist? Is existence a property, like being red or being heavy? Kant argued it is not. Existence adds nothing to a concept. A hundred real dollars have the same properties as a hundred possible dollars, but they are not the same in every respect. The difference is that one exists and the other does not. But that difference is not a property in the ordinary sense.

Philosophers have debated whether existence is a predicate, whether it comes in degrees, and whether abstract objects like numbers and propositions exist. Plato thought they did, in a realm of Forms. Nominalists deny it, holding that only particulars exist. Existentialists focus on existence as lived. For Kierkegaard and Sartre, existence is not a neutral fact but a task. You do not just exist; you have to decide what to do with your existence. Heidegger distinguished existence from mere presence. Only humans exist in the full sense, because only humans are aware of their own being.

Questions about existence

• Is existence a property? Does adding "exists" tell us anything about a thing?
• What exists? Physical objects, minds, numbers, universals?
• Degrees of existence. Do some things exist more fully than others?
• Existential meaning. What does it mean to exist as a human being?

Existence is the most basic fact and the hardest to define. It is presupposed by every other inquiry.