In the last blog, we explored the philosophical tool of modal logic known as possible worlds. Basically, those things that are possible are said to “exist” (the conditions are said to be true) in some possible world. Here, we continue our presentation of the modal ontological argument for God’s existence.
What about Impossibility and Necessity?
Since something is possible if those conditions are true in some possible world, we can understand something to be impossible if those conditions are true in no possible world. As you know, it is impossible for someone to be a married bachelor. A person cannot be both married and not married at the same time and in the same sense. It is also impossible for someone to draw three straight lines, on a two-dimensional plane, each one perpendicular to the other (You can try right now. On a piece of paper, draw two lines at a 90o angle to each other, and then draw a third line that is angled 90o from the other two… good luck!). There are no possible worlds in which these things exist, and so they are said to be impossible.
Likewise, something is necessary if it is true in every possible world. The equation 2+2=4 is necessarily true because it is true in every possible world. There is no possible world in which this equation is false. The Latin phrase ex nihilo nihil fit, “out of nothing, nothing comes,” also falls into this category, since it is a metaphysical truth that governs how reality works. Therefore, things that are necessary are a subset of things that are possible. After all, in order for something to be necessary, it must also be possible. But not everything that is possible is necessary.
To illustrate it visually, imagine a clear bowl of water surrounded by numerous vials of liquid dye. Each H2O molecule would represent a possible world, one of which would also be the actual world, and each color of dye would represent a condition of reality that was either possible, impossible, or necessary. Suppose you picked up some green dye which was labeled “the creation does not exist.” If you poured it into the bowl, you would see a confined cloud of green water. This illustrates something that is possibly true. If you poured some green dye labeled “out of nothing, nothing comes” you would see the whole bowl turn green, because that is a necessary truth. Finally, if you poured some red dye into the bowl that was labeled “John is a married bachelor,” you would only see crystal clear water, because that statement is impossible.
Two Different Kinds of Possibility
When thinking about possibility and impossibility, it is important to distinguish between something called metaphysical possibility and epistemic possibility. For this, please observe the following equation:
(√784 – 18)2 = 34 + 19
In your opinion, what is the equation’s truth value? Assuming you didn’t plug it into a calculator, your temptation might be to say, “it is possible the equation is true, and it is possible that it’s not true.” And, in a manner of speaking, you would be correct. Nevertheless, that would only be correct in terms of epistemic possibility. That is, it is possibly true or false for all you know. But in a stronger sense, a metaphysical sense, the equation is either true or false regardless of what you know. So, epistemic possibility only concerns what we know, but metaphysical possibility has to do with what is really possible. 
With reference to the equation above, it is possibly true or false for all we know (epistemic), but is really only true or false (metaphysical). When applied to the question of God’s existence, many people stop short by thinking epistemically and conclude that it is possible for God to exist or to not exist. However, when talking about modal possibility (i.e. possible worlds), we are always talking about metaphysical possibility.
Therefore, when we look at the actual argument in the next blog, keep in mind the foundation that these first two blogs have laid. To assert that something exists in some possible world is to say that those conditions really are true in some possible world, not that they could be true for all we know. Further, we are never concerned with mere epistemic possibility, but always with metaphysical possibility.
 Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics, third edition, p. 185.