What does it mean to say something exists? What exactly is existence? How does it differ from non-existence?
Most of us have a common-sense notion of existence, and thus we have never really thought these questions through. But as with so many other things, we know what existence is until we are asked to define it! Admittedly I’ve never woken up at 4:00 in the morning in cold sweats trying to figure out what existence is and how it differs from non-existence, but it is fun to think about nonetheless.
What is existence, then? Does saying something exists mean it located in space and time? If so God could not exist. In fact, space and time themselves could not exist because space is not located in space, and time is not located in time.
Is existence defined by physicality? If so, what about numbers, relations, properties, sets, acts of knowing, intentions, motives? None of these are physical, and yet they clearly exist.
Is existence defined as whatever is created by God? Since God did not create Himself, He must not exist according to this definition. Furthermore, things such as logic, numbers, and relations are not created entities. They exist eternally in the mind of God as part of His rational nature. None of these definitions will work.
So what is existence? Existence could be said to be thatness. It’s that which makes a real difference in the world. It differs from non-existence in that existence exemplifies at least one property, while non-existence exemplifies no properties whatsoever. It might be said that existence is the entering into the exemplification relation. “Existence is either the belonging of some property or the being belonged to by a property.”1 Consider human existence. The fact that the property of being a human belongs to something and that something has this property belonging to it is what confers existence.
Existence can be had (in the case of God He has always had it), gained (in the case of everything but God and the concepts inherent to His rational mind), or lost (in the case of many material things), but it cannot be had in degree, gained in degree, or lost in degree. Coming-to-be or ceasing-to-be are not gradual processes like walking into a room, in which case there is a point at which you begin to enter, enter half-way, and then finally come to be in the room. Existence either is or it isn’t.
1 J.P. Moreland and William Lane Craig, Philosophical Foundations for a Christian Worldview, 187-193, 202.