Teleological reasoning was common among authors of the XIII century. Certainly, the existence of a finalist order among things allowed them to explain both the movement of natural bodies and the movement of the celestial bodies: for these authors all things would move because of final causality. Aquinas' Summa contra gentiles, which we analyze in the following, reproduces this same reasoning model. Taking as reference the movement of natural bodies, he tries to explain the meaning of a special category of movement, namely: human knowledge. Thus, he states that human knowledge is an expression of a natural appetite of our intelligence, the natural desire to know, which rests only in the knowledge of God, the first cause of the world.