Now it seems that everything in the world stems from sources other than God, since the products of nature have their source in nature; deliberate effects can be traced back to human reason or will as their source. There is no need then to assume that God exists.
Thomas AquinasIf all the sins of the flesh are worthy of condemnation because by them man allows himself to be dominated by that which he has of the animal nature, much more deserving of condemnation are the sins against nature by which man degrades his own animal nature.
Thomas AquinasWhat can be accomplished by a few principles is not effected by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle, which is nature, and all voluntary things can be reduced to one principle, which is human reason, or will. Therefore there is no need to suppose God's existence.
Thomas Aquinas