Saint Thomas 5 ways to prove the existence of God

Saint Thomas Aquinas (1225–1274) was an Italian philosopher and theologian. He is considered the father of scholasticism, a philosophical current that tried to apply ancient Greek philosophy to the understanding of Christian doctrine.


Saint Thomas and the rational justification of faith

The thought of Saint Thomas belongs to a tradition that reflects on Christian doctrine in the light of Greek philosophy, trying to give religion a rational foundation.

Much of medieval European thought started from considering reason as a servant of theology ( Philosophia ancilla theologiae ), in the service of demonstrating truths that were unprovable from the perspective of faith. With this objective, Saint Thomas will recover the thought of Aristotle, which had hardly had any influence on European culture in the first centuries of our era, because it was not translated from the Greek. Until now, it had been Plato ‘s doctrine that had been used to justify faith philosophically.

A milestone in this attempt at rational justification of faith had been the so-called “ontological proof” of Saint Anselm, an eleventh-century monk who tried to explain the existence of God by appealing to reason. For Saint Anselm, in the idea that we have of God there is implied an awareness of the “need of him” of him. That is to say, while some imaginary entities, such as unicorns, we can think of as non-existent, we have to conceive of God as existing, because he harbors in himself all possible virtues, including existence. If we do not conceive it as existing, we could have in our imagination another being, even more perfect, that would exist. And that being would then be God.

Summa Theologica Minima, by Thomas Aquinas, edited by Peter Kreeft (Technos)


In the Summa Theologica , the treatise in which St. Thomas unfolds an important part of his thought, he bases himself on the concept of the Aristotelian « immovable motor

 » to unfold a rational justification for the existence of God.

For Aristotle, the motionless mover was the guiding principle of the universe. He speaks of it in his treatise on Metaphysics and describes it as that which is the beginning of everything and which, in turn, cannot have a beginning. The immobile motor is also a moral principle: it amalgamates all possible virtues and the particular entities of the world try to approach it through those virtues (such as goodness, beauty or justice). It is the human being, due to his nature as a rational animal, who can be closer to the immobile motor because he can know it rationally.


But let’s see what these five ways are by which Saint Thomas argues the existence of God and by which, in addition, we can know some of his attributes.

The 5 ways of Saint Thomas

Path of movement

Santo Tomás takes the distinction that makes Aristotle of potency and act . Something that is in potential has the ability to become something else. For example: a seed is a potential tree. When the seed grows and actually becomes a tree, we say that it is a tree in act . Movement, in this scheme, is the passage from power to act.

For Saint Thomas, what moves (that is, what goes from being in power to being in action) is because something or someone moves it. There is some kind of impulse external to it that sets it off. For example, the seed needs water, soil, and sunlight to grow into a tree. In turn, that which moves the seed and everything that moves has had to be moved by something else: the earth has not always been earth, the water has passed through different states, the insects that contribute to the reproduction of the plants one day were larvae…

Everything that moves is moved by something external, and at the same time these external entities are moved by others. We could go further back in a series of infinite causes to the prime mover of everything. But since we cannot go back to infinity (according to the thought of the medieval philosopher), there must be at some point an initial motor that sets the whole movement in motion.

That engine is also immobile. That is, it is already in action in all possible senses. This is so because it is impossible, for St. Thomas, for something to be a motor and to move at the same time, just as it is impossible for something to move itself, finding within it everything necessary to go from potency to act. Therefore, the motor must be stationary.

«It is necessary to reach a first motor that is not moved by anyone, and this is the one that everyone understands by God»

Path of efficient cause

It is another Aristotelian concept that refers to the cause by which a certain effect is produced in the world. For example, the cause of a wooden table is the carpenter who makes it. Just as it happened with necessity, each efficient cause has, in turn, another efficient cause (we are the cause of our parents, our parents are the cause of our grandparents…).


Since, furthermore, for Saint Thomas it cannot be that an efficient cause is the cause of itself, and since we cannot go back in an infinite series of causes because then there would not be a first cause that would cause the others, there must be an efficient first cause.

That first cause is the cause of everything else and has, in turn, no cause. And for Saint Thomas that cause is God.

Path of contingency and necessity

Beings are born and die, because they may or may not exist, and the world continues to function and be the same. For Saint Thomas, “contingency” refers to that condition of beings and is opposed to “necessity”.

The necessary beings cannot not exist, the only possibility is that they exist. Hence their necessity, since they are necessarily existent . It is impossible, in Aristotelian reasoning, that there have always been contingent beings. Because precisely because of their contingency there was a moment when, with all certainty, they did not exist.

So how do you go from not-being to being? If all things were contingent, then there was a time when nothing existed and, therefore, none would exist now, since some are causes of others. That is, if all beings were born and died, there would have been some time when nothing existed.

There must be, then, some being that has always existed, whose necessity depends on itself and who is the cause of the necessity of the other necessary things. That is what we call God.


Way of the degrees of perfection

Saint Thomas starts from considering that the things of the world have attributes, to a greater or lesser extent. Some things are beautiful, and more or less beautiful than others, for example. All things in the world more or less approach perfection in these attributes.

This implies that there must be a model with respect to which to establish a comparison between things by their attributes: a model of maximum beauty, or goodness or justice. For Saint Thomas, this model is an optimal being, maximum in all degrees of perfection, with respect to which to establish comparisons.

That supreme being that amalgamates all the virtues, and whose degrees of perfection allow comparisons of degree between the beings of the world, is, for Saint Thomas, God.

«There exists, therefore, something that is for all things the cause of their being, of their goodness and of all their perfections, and this we call God»

Path of purpose

All beings have a purpose. For example, the purpose of the knife is to cut correctly. But inanimate beings, who lack knowledge, cannot reach their end if it is not because an intelligent creature pushes them to it. For example, we are the human beings who, using knives, lead them to fulfill their purpose.

Intelligent beings tend to higher ends, but they also act according to a purpose. And in the same way they need a higher intelligence that guides them and leads them towards the fulfillment of their purpose. This elevated intelligence, which directs all the others towards their purpose, is what we call God.

As can be seen, the five paths not only offer a justification for the existence of God, but also advance some of his attributes. God is thus described as an immobile motor, that moves everything without being moved, the cause of everything else, being necessary, being extremely perfect and supreme intelligence.

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