The three characters in the Dialogues represent three very different attitudes towards how we can gain knowledge of God. Demea believes that God is beyond our comprehension, that there is no way we will ever understand him beyond the most rudimentary level. Philo is a sceptic, and as such is dubious about arguments a posteriori, preferring a priori. Cleanthes on the other hand is an empiricist, and prefers to base his arguments upon the evidence that he can gather from the world. Thus, it is not surprising that this theory that he proposes relies heavily upon the observed features of the world. He begins by noting "the curious adapting of means to ends" in the universe at large, and then goes on to note that "human contrivances" also exhibit this feature. The universe is one almost infinitely subdividable machine, composed of smaller machines, as one can say a human machine is. From this comparison, he moves on to say that the universe is, in fact, analogous to a machine. This is useful because it allows us to apply information we have about machines to the nature of the universe. One such important fact, and indeed the pertinent one that Cleanthes has postulated this reasoning in order to obtain, is that human machines have intelligent designers. If we then follow his analogy, the universe would have an intelligent designer, albeit of a much grander scale. In fact, he believes that we can go further and not just claim that the designer possesses intelligence, but in fact possesses human-like intelligence. This allows him to explain the nature of God, which is his purpose.
Cleanthes having proposed this, Philo's instant response, as a sceptic, is to note the inductive reasoning that this requires, and to note the weakness of inductive reasoning. However, Cleanthes has dealt with the objections to inductive reasoning to a large extent earlier on, where he accuses all sceptics of not actually being sceptics, since if they were they would have no reason to follow the basic laws of physics, or indeed of anything. He pokes fun at Philo by saying
``We shall then see, whether you go out at the door or the window'',
since if he is a total sceptic there would be no reason for him not to do the latter, but of course that is absurd, as it seems plain to most that he would of course fall. Scepticism is useful to highlight that inductive reasoning is fallible, but it is no reason to reject it. However, a weaker form of this sceptical reasoning could be quite damaging to the analogy argument, since one must ask, and Philo indeed does ask, how similar these two 'analogous' items are. It would seem that there is a good deal of difference between the universe and some human machine, if for no other reason than scale. We are not merely considering a moderate scaling up of the idea, the latter is in fact some insignificant speck contained within the former, so this near infinite change of scale seems to greatly weaken the analogy, and thus the strength of the conclusions that we can draw using it. The analogy is also weakened by the severely limited nature of the data input into it. That is to say, we are basing a generalisation about the universe on what we have observed in our portion of it, which we have little reason to accept as the standard format of the rest of the universe. Should the rest of the universe turn out to be totally disordered and chaotic, then the resemblance of the universe to a machine would be almost non-existent. There simply is not enough experimental data to make such a sweeping statement.
As well as these obstacles to be overcome, which are not fatal, merely problematic, there are in fact several points that actively indicate against the analogy. In a machine, every part has a purpose, and failing to fulfil that purpose leads to problems for the machine as a whole. One cannot say the same of the universe. Many things exist that serve no purpose at all. One example is the appendix. If we were designed, and Cleanthes uses the example of the eye as evidence for design, why would we need some vestigial organ whose only action is to become infected and cause pain and potential death? This bodes badly for our designer, leaving in malfunctioning parts. Indeed, if we were created in God's image, it would suggest that he contains such an organ, making him less than perfect. A more widespread example of something with unnecessary existence is what is termed 'natural evil', that is natural phenomena that cause suffering. If the world was so well designed, why is it that earthquakes tear it apart? It would seem that if the universe is indeed a machine, it is a very poorly designed one, and this is surely not the God that Cleanthes would like to demonstrate.
Cleanthes second premise in arguing the analogy is that machines have intelligent designers, and therefore, since the universe is like a machine, it has an intelligent designer. What he is really saying, as Philo points out, is that it requires reason as the source of motion to bring order in to existence. Philo goes on to attack this idea, pointing to animals and plants as examples of how order can be imposed without intelligence. He goes further, and suggests that perhaps the universe would be better compared to an animal or vegetable than a machine. In that case, by analogy, there would be no need for an intelligent designer, and no need for a God. However, this argument is still open to all the previous criticisms of analogous arguments.
An alternative view is expounded by Philo, one that he attributes to Epicurus. If one supposes time infinite, and matter finite, then every combination of matter will occur. What may be the case is that ordered systems are more stable than disordered ones, and therefore will remain coherent for longer. This would lead to a more ordered universe, potentially like the one we see today. A major problem with this is the idea of entropy, which may not have been available to Hume. This states that everything in fact tends the other way, to disorder. This would lead to a totally disordered universe, and most would agree that that is not what we have in existence currently. Entropy does however admit one small possibility. If enough disorder is created, then some order can be created, since the net effect is disordered. Thus, if the rest of the universe is chaotic, as was admitted as a possibility earlier, then it is allowable that our section could become ordered. In this manner, the need for a creator is eliminated.
Even if Cleanthes was able to surmount these seemingly insuperable problems, he would have yet another set to face. He is trying to extrapolate a Christian style God from this data, but it does not lend itself to this. What Cleanthes wants is an omnipotent, omniscient, benevolent God. What he is allowed by his own argument is very different. One of the earlier assertions that Cleanthes makes is that God must in fact have a human type mind, since we know that machines have human type minds behind them. However, that would suggest that God is not infinite, or omnipotent, or any of the things that Cleanthes tries to imply. The analogy also admits the possibility that since the designers of our machines are finite, God could well be. In fact, if he is more human like, we are lead into a collaborative pantheon. Demea is shocked by this, as is Cleanthes, but it is nonetheless present in the argument.
If we take the quality of the workmanship as a good indicator of the quality of the workman, then this too leads to a weak God, since there are many flaws in his universe, as have already been indicated by its lack of similarity to a perfectly functioning machine. If God is not omnipotent, then we have to ask exactly how potent is he. Whatever answer less than totally is not going to reconcile with a Christian God.
It seems then that Cleanthes' design theory is not going to produce the desired results, if it can stand up at all. It seems that analogous reasoning is not very reliable, that the analogy is not particularly strong in the first place, and also that unpleasant conclusions are unavoidably created by the theory. Should we accept his argument we can no longer have any justified belief in a Christian God, but perhaps this is the point. Just because Cleanthes set out searching for a Christian God is no reason to suppose he will find one. It may be the case that this argument does in fact lead us to the true creator of the universe, but the flaws in the basis of the argument suggest against that. I would then suggest that Cleanthes must abandon this theory, it having almost no redeeming features. It is based upon false premises, and thus, unsurprisingly, leads to false conclusions.
J. Cottingham, Western Philosophy: An Anthology (Oxford: Blackwell Publishers, 1997) David Hume, Dialogues Concerning Natural Religion (London: Penguin Books, 1990)