In this section, Philo develops his analogy between the universe and an animal body. If the universe is like an animal or vegetable body, it is possible that it did not come into being by intelligent design. Philo spends this section trying to show that these two theories are every bit as plausible as the theory that the universe was produced by design. His objective here is the same as in the last chapter: if he can show that both of these operations are just as plausible candidates for the cosmogonic principle as the design principle is, then he can show that there is no reason to think that the universe really arose through design.
Though it is difficult to understand how generation or vegetation could have given rise to the world, he begins, it is no more difficult to understand how these two processes could have given rise to the world than it is to understand how design could have given rise to the world. In all three cases we can tell a convoluted, not very illuminating story. In the case of vegetation, we could say that just as trees shed seeds, which then sprout other trees, so too the planetary system creates seeds within itself which then sprout material planets. Maybe, he conjectures, comets are these seeds. In the case of generation we might say that the comet is like an egg, which hatches into an ordered universe.
Philo admits that he cannot make explicit sense of either of these pictures, but, he points out, neither can Cleanthes make any explicit sense of the claim that the universe is the result of divine design. Philo's point here is not that these two alternative cosmogonies are actually more accurate representations of what really happened, but rather that all three pictures are just ineffectual metaphors. To say that the world was created through generation, vegetation, or design is really just to refer to whatever incomprehensible cause is responsible for the known effects we see around us. We choose to talk about the origins of the world in terms of these processes because these processes are familiar to us. But these are obviously not the only possible processes through which ordered systems can originate; they are simply the three principles of origination that we happen to find in our small corner of the universe. To think of these processes as anything other than convenient placeholders for unknown processes, therefore, is to delude our selves into thinking that what we know about the world is all there is to know.
After Philo finishes presenting this latest thought, Demea unexpectedly steps in on the side of the design argument: if the world really did have this property of being able to sow its own seeds of propagation, he remarks, that would be a great argument for God as designer. After all, a self-propagating system is an amazing feat of design and so it really calls out for some purposeful agent as its cause. Philo has two responses to this objection. First he reminds Demea that he has already shown that order is no more a natural property of thought than of matter, and therefore there is no reason to think that just because there is an amazing system in place, intelligence must have put it there. The case of a self-propagating universe is the same as the case of a tree or a duck: we cannot really understand how this order comes about, but it could just as easily be because of some principle of order inherent in the matter as it could be because of some principle of order inherent in thought. We simply do not know what produces order, and to think that all order must be the result of intelligence is to make an unfounded assumption.
Philo also points out that even if this objection were legitimate (which it is not), Cleanthes could not use it. If Cleanthes demands to know the cause of Philo's vegetative principle, then Philo will demand to know the cause of Cleanthes' reasoning principle. And Philo even has the better footing here because we always see reason arise from generation but never vice versa. In other words, if Cleanthes wants to infer design from Philo's generation, then Philo can just turn around and insist on inferring generation from Cleanthes' divine intelligence: he can say that the intelligence that designed the universe must have been born at some point since all minds are born at some point. He would thus make generation the ultimate cause of the universe instead of design.
In this section, Philo appeals to what we might call an "objection from the narrowness of the evidence": he accuses Cleanthes of trying to draw conclusions about the entire universe from observations made in one narrow corner thereof. This kind of objection could be made if you know of only one of your brother's childhood friends and thereby assume that every time he mentions something about a childhood friend he is talking about that specific one. Philo believes that Cleanthes' reasoning runs along similar lines: in our narrow experience we observe only one operation that produces ordered systems (intelligent design), therefore that must be the only such operation in existence. Philo corrects Cleanthes first by showing that even in his narrow store of experience there are two other such processes that produce ordered systems: generation and vegetation. But his main point here is that we have no reason to think that any of these three operations is the operation responsible for producing the world. These are simply the three such operations we happen to know about; but our lack of knowledge in no way means that they are the only such operations that exist.
However, Cleanthes has an opening for reply here. Newton, for instance, observed gravitational effects in one small corner of the universe (earth) and from there drew conclusions about the entire universe. The same can be said about all of the scientific conclusions that have been drawn about the nature of matter and the properties of light: scientists only observed light and matter in one small corner of the universe, but they claim to thus reach knowledge about all matter and light. Scientific reasoning, then, seems to engage in the same form of "mistake" that Cleanthes engages in. Scientists constantly extrapolate from the evidence they find in their small corner of the world. Given that this sort of reasoning is standard in science, it seems difficult to fault Cleanthes for engaging in it.