Full Idea
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 God does not exist.
Gist of Idea
Non-human things are explicable naturally, and voluntary things by the will, so God is not needed
Source
Thomas Aquinas (Summa Theologicae [1265], Ia,Q02,Art3,Ob2)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.82
A Reaction
Not, of course, the opinion of Aquinas. So the possibility of naturalism (assuming the human will can be further reduced to nature) was a clear option in the thirteenth century. In reply Aquinas cites his Fifth Way.
Related Idea
Idea 21273 Way 5: mindless things act towards an obvious end, so there is an intelligent director [Aquinas]