Imagine a being with unlimited power, unlimited knowledge, and perfect moral goodness. Call that being maximally excellent.
Now add one more requirement. Maximal greatness means having that excellence in every way reality could have been, not by luck and not only under favorable conditions. A being that was all-powerful in our world but weak, or absent, in some other way things could have gone would be very great, but not maximally great.
Notice what follows from this. A being can’t be powerful, knowing, or good in a reality where it doesn’t exist. So to be excellent in every possible reality, it would have to exist in every possible reality.
Nothing here claims that such a being exists. This step only says what we mean by the words.
Some ideas contain a contradiction and so can’t be true under any circumstances, like a square circle or a married bachelor. The idea of a maximally great being doesn’t obviously contain one. Unlimited power, unlimited knowledge, and perfect goodness don’t clash the way “square” and “circle” do.
Notice what this step does not say. It doesn’t say we can prove such a being exists, and it doesn’t say we can picture one. It says only that the idea is genuinely possible: that among all the ways reality might have been, at least one contains this being.
Suffering in the world, and the lack of direct evidence for God, raise real questions about whether such a being exists. But a reason to doubt that something exists is different from a reason to think it couldn’t exist. Some philosophers argue that evil shows the idea is incoherent, and others have answered that argument at length. This page doesn’t settle that dispute.
This is the premise the whole argument depends on. The lack of an obvious contradiction is a reason to take the possibility seriously, but it is not proof.
Philosophers use the phrase “possible world” for a complete way reality could have been. These aren’t alternate universes floating somewhere out there. They’re descriptions of how everything might have turned out.
If something is genuinely possible, then at least one of those complete descriptions includes it. So if a maximally great being is possible, there is a possible world in which it exists.
Take a world where a maximally great being exists. In that world it is maximally great. But by Step 1, being maximally great means being maximally excellent in every possible world. So it exists, and is excellent, in all of them.
This is not defining God into existence. The definition says only what such a being would be like. Whether any being actually fits it is exactly the question Step 2 asked.
Of all the ways reality could have been, exactly one is the way it actually is. That is the world we live in, so it is among the possible worlds.
If a maximally great being exists in every possible world, it exists in ours.
If a maximally great being is genuinely possible, then a maximally great being actually exists.
This is a strange feature of the idea. For most things, being possible doesn’t make them real. A maximally great being would be the exception, because its existence would be either impossible or necessary, with nothing in between.