Just One Pure Act

How Aquinas's classical theism secures monotheism

One attractive feature of Aquinas’s notion of God as pure act is that it secures an
in principle” monotheism. That is, within Aquinas’s metaphysic, if there is a being that is pure act, there can only be one by metaphysical necessity.

Here’s how to see this. Assuming a being of pure act is a being of pure existence whose nature is simple and self-subsistent (this follows under Aquinas’s metaphysic), such a nature precludes the possibility of multiplication since multiplication would entail having to contradict its nature as either simple or self-subsistent.

To draw this out, let us suppose there could be something whose essence just is its existence. This something would be simply ‘Exists!’ and thus ontologically simple. Now, if there are two instances of ‘Exists!’ there is some difference between them. (Note: even assuming the identify of indiscernibility principle is false such that these qualities or properties might be indistinguishable, nevertheless, trivially so, there is still a difference between two instances of something.) However, by the fact of there being two instances, this would imply that even the similarity between the two instances be expressible in terms of qualities and properties, and that such expressions are ontologically grounded. This contradicts the simplicity of pure existence, since to say that each instance has one or more quality or property, is just to say that it is ontologically complex. So, God as pure act is in principle non-multipliable. There can be only one.

For further development of this argument, and others extracting the classical theistic divine attributes from the notion of pure existence, see my book The Best Argument for God.