As with Paley’s Design argument,Aquinas’s third way is a posteriori and inductive, so it is based on observation. In Aquinas’s case this is the observation that the universe exists. Aquinas’s used the feelings of sight to verify the existence of the Cosmos. In Aquinas’s argument known as way three, all the observations and all things that we can see in the universe are Contigent, meaning they rely on each other for their existence. And Aquinas observed that all things in the Universe are contingent, they are moved,changed,and caused, they need not exists but they all do. And this applies to galaxies,stars,planets,people, and trees. Even galaxies can collide, with immeasurable change and consequence. Stars can explode and create new stars from their debris. All the heavy elements come from such explosions, including those which make up your body. From this observation that all things are contingent Aquinas concluded that something must exist necessarily, meaning it exists eternally without the need of anything else. This is because if everything that exist is contingent, then the cause of the universe would seem to lie outside it. There seems to be nothing in what we observe that can explain why contingent things exist. The Cosmological Argument therefore deduces from this that this external reason must itself be necessary.
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