Christopher Martin provides an interpretation of the fifth way which I find rather plausible. He contends that “unconscious teleology is always relative to a system, is always a case of a part being for the sake of the whole” (1997, 200). Furthermore, he argues that if A is for the sake of B, and B is for the sake of C, but C is for nothing whatsoever, then A and B are also (in reality, initial appearances notwithstanding) for nothing whatsoever. As Martin puts it:
If l put my file on the floor, and then a book on top of it, and then my word-processor on top of the book, and then a flower-vase on top of the word-processor, and then put a stick in the flower vase, and then carefully wrap a small scrap of ribbon around the stick, a question arises of what the devil I'm up to; more scientifically, what is the point of all that? [...] If the last step of tying the ribbon, or the system as a whole, fails to have a point, one is surely entitled to say that the apparent point of each previous stage is wholly illusory. (Ibid., 196-197)
The fifth way then goes as follows: every instance of unconscious teleology is system-relative; the world is the all-encompassing system which includes all unconscious teleology. If the unconscious parts of the world are to have teleology, the world itself must have teleology (see Martin’s point above). “If the world has a point, if it is for anything, this cannot be a case of unconscious teleology, since unconscious teleology is always relative to a system, is always a case of a part being for the sake of the whole. There is, by definition, no greater whole of which the world forms a part. [...] Therefore the teleology of the world must be conscious: the point of the world must be conferred on it by some mind. ‘Therefore there is some being with understanding which directs all things to their end, and this, we say, is God.’” (ibid., 200). I think this is pretty convincing.
