<p>Some ontological arguments for the existence of God (coming from an atheist):</p>
<p>Australian philosopher Douglas Gasking:
1. The creation of the world is the most marvelous achievement imaginable.
2. The merit of an achievement is the product of (a) its intrinsic quality, and b) the ability of its creator.
3. The greater the disability (or handicap) of the creator, the more impressive the achievement.
4. The most formidable handicap for a creator would be non-existence.
5. Therefore if we suppose that the universe is the product of an existent creator we can conceive a greater being ? namely, one who created everything while not existing.
6. Therefore God does not exist.</p>
<p>David Hume:
1. The only way to prove anything a priori is through an opposite contradiction. For example, I am a married bachelor.
2. The resulting contradiction makes something inconceivable. Obviously it is impossible to have a married bachelor.
3. It is possible to comprehend anything not existing. Thus it is not inconceivable to imagine anything not existing.
4. Nothing can be proven to exist a priori, including God.</p>
<p>Descartes:</p>
<ol>
<li>I exist</li>
<li>I have an idea of a supremely perfect being, i.e. a being having all perfections.</li>
<li>As an imperfect being I would be unable to create such a concept through my own thoughts.</li>
<li>The concept must have come from God.</li>
<li>To be a perfect being God must exist.</li>
<li>God exists.</li>
</ol>