When faced with the experience of reality, the fact that things exist (whatever their nature may be), we are presented with the irrefutable fact that insofar as they have an act of reality they are not also their opposite. Ontologically speaking, we do not have a situation where a thing exists and does not exist at the same time. In other-words the very act of reality is fundamentally intelligible.
This is where intelligibility fundamentally begins for metaphysics. It is the evidence that we can apply the principle of non-contradiction to the act of reality universally.
Insofar this is true we can make inferences about what cannot be true in reality, such as we cannot have a square circle in reality, and sometimes by inferring what cannot be true in reality we can also know what must be true under pain of absurdity. But we cannot necessarily know particulars with this method of inference. I cannot know if a particular kind of atom exists, and as such this qeustion would be a job for science since metaphysics can only know very general truths, or if we come to know a particular thing it is because it follows necessarily from what is generally understood.
2. Ontological Causality.
When talking about cause and effect in metaphysics people are not speaking of it in the same sense that science does. The person doing metaphysics is not concerned with the particular relationships between physical things or particularly what they are, but rather is concerned with the relationship between possibility/potentiality and the beingness of any nature.
Because the principle of non-contradiction applies universally, one can know that if there were absolutely nothing it would be metaphysically impossible for something to begin existing for the simple fact that there is absolutely nothing. Because of this it must be true that there has to already be an actual reality before any possibility or potential can become an actual reality. This is what the person doing metaphysics means by cause and effect, and may use terms like “existential cause” or “ontological cause” and may proceed to make categorical distinctions between a things essence and esse like Aquinas.
3. Proving God’s existence/The uncaused-cause.
It’s on this basis that people like Aristotle and Aquinas use the principle of non contradiction and reductio ad absurdum to build a genuine system of knowledge that does not conflict or undermine any other system of knowledge such as physics/science. Metaphysics stands for beyond or after physics, and is not itself a study of physics but rather it is the study of what it means for a thing to exist…