Throughout his life, Aquinas wrote an incredible number of philosophical texts that touched on many different subjects, ranging anywhere from natural philosophy and the work of Aristotle to theology and the Bible. His most famous and extensive work, Summa Theologiae, provides the most detail in terms of Aquinas’s philosophical views. Aquinas began Summa Theologiae sometime after 1265 and wrote it until his death in 1274.
Summa Theologiae is broken up into three parts, with each part featuring its own subdivisions. It is in Part 1 that Aquinas’s most famous philosophical text, the Five Ways, is found. In this, Thomas Aquinas sets out to prove the existence of God.
Aquinas begins by acknowledging that though philosophy is not a requirement in promoting God’s knowledge, it can help theology. He then tries to answer the following questions:
- Is “God exists” self-evident?
- Can this be demonstrated?
- Does God exist?
Aquinas then provides five proofs that show the existence of God. With his Five Ways, Thomas Aquinas combines the ideas of theology with rational thought and observations from the natural world, in order to prove the existence of God.
We can see that there are things in this world that are in motion. Anything that is in motion was put in motion by something else that was in motion. And that object is in motion because it was put into motion by another object that was in motion, and so on and so forth. However, this cannot infinitely keep going backward because there would never be an original mover (and thus, there would never be the subsequent movement). So there must be an unmoved mover that is first, and that is understood to be God.
Everything is caused by something, and nothing can be caused by itself. Every cause is the result of a previous cause, and that previous cause was the result of another previous cause. This cannot infinitely keep going backward because if there is no initial cause, then there are no subsequent causes. So there must be an uncaused first cause, which is understood to be God.
We observe in nature that things come to exist and then cease to exist. However, everything that exists needs to come from something that exists, and if it is possible for something to not exist, then it wouldn’t exist before, and it wouldn’t exist now. So there must be a being whose existence does not rely on the existence of others, and this is understood to be God.
We observe that beings have varying degrees of characteristics (more good, less good, more noble, less noble, etc.). These varying degrees are being compared to a maximum (the noblest, the best, etc.), and according to Aristotle, the greatest state of being is when there is the greatest state of truth (the maximum). So there has to be a cause to the perfections we find in beings, and this perfection or maximum is understood to be God.
We observe unintelligent and inanimate objects in nature acting toward a purpose, even if these objects are not aware of this fact (such as the food chain or the processes of sensory organs). Though unaware, these objects are clearly acting toward a purpose according to a specific plan, and therefore, there must be a being guiding them that has the knowledge to direct them toward their purpose. This is understood to be God.