What was Ibn al-Qayyim's proof that god exists?

Ibn al-Qayyim wrote about a watermill by a river, faultlessly made, with perfect parts: no flaw can be observed in its construction. It efficiently irrigates a large garden containing various kinds of plants and fruits. The garden is well tended, pruned, weeded, and maintained in every way so that nothing is amiss or overlooked; and nor is any fruit left to rot. Then its produce is harvested and the money gained is distributed to various people according to their needs, each getting what is right for them. All of this happens each time, over and over again, without fail. Would you say that all of this happens by chance, without someone behind it who has intentionality (iradah), a will to choose to do or not to do (ikhtiyar), and the ability to plan and manage (tadbir)? Would you believe that the wheel or the garden got there by mere chance, or that all that goes on there does so without an actor who has intentionality, will or management? What would your intellect say to that? What would that indicate to you? (Miftah Dar al-Sa‘adah, 2:69-70.)