Three of Philo's main lines of attack rest on the claim that there is simply not enough evidence in nature to allow the empirical theist to draw the conclusions he wishes to draw. The first line of attack shows that there is nothing in the natural evidence to indicate that God has any of the natural attributes typically ascribed to him. Allowing, for the sake of argument, that the world is a machine and that its cause must be some intelligent designer, there is nothing about this machine to indicate that the designer is infinite, perfect, one in number, or incorporeal. For all the evidence tells us our God could be stupid, could have a body, or could even be a team of minor deities working in unison. A similar line of attack is used for God's moral attributes. For all the evidence tells us, God might not even be very nice.
Philo also uses the lack of evidence in a third line of attack. He attempts to show that the evidence available to us from the natural world could actually support any number of analogies equally well, not just an analogy to a machine. For instance, the evidence would support an analogy between the universe and an animal or even a vegetable.
The argument from design is supposed to prove that the universe was created by something that resembles human intelligence. The argument would thus establish two important points of theology. First, it would prove that there is a God, the intelligent designer behind the universe. Second, it would establish God's nature as somewhat akin to our own, by claiming that divine intelligence works similarly to human intelligence.
The first step in the argument is to establish that the universe highly resembles a machine: The universe appears to be highly ordered and perfectly adapted to the wellfare of all its inhabitants; each part from the smallest to the largest works harmoniously for the good of the whole.
Next, we must gather what we know about machines. We know that every time we have ever come across a machine it has been made by an intelligent designer. We have had so much experience of this conjunction (i.e. of machine with man as its cause for existence) that if we were to stumble upon any machine, even somewhere out in the wilderness far from any civilization, we would immediately and justifiably conclude that the machine was created by a man.
Finally, we must put together the analogy and our knowledge concerning machines and draw a conclusion about the universe: We are justified in believing that all machines are created by an intelligent designer. The universe seems to be a machine as well. Therefore, we are justified in believing that the universe was created by an intelligent designer, i.e. God.
How does Philo use the problem of evil to argue against empirical theism?
Philo uses the problem of evil in an atypical way. Instead of using it to argue that there is an inconsistency in the traditional conception of God, he uses it to show that we cannot infer God's moral attributes from the natural world. The world around us appears to be far less perfect than it could be. It is filled with evil and misery. It might well be the case, he admits, that the world is actually as perfect as it could possibly be, or that there is some reason why it needs to be the way it is. But the fact remains that when we look at the world with all its evil and misery, we have absolutely no grounds on which to conclude that God is supremely good, powerful, and wise. If he is supremely good then he would not want there to be evil. If he is supremely powerful then it would be within his power to will evil away. And if he were supremely wise he would know how the make the world perfect for its inhabitants. Looking at the world, then, we cannot reasonably conclude that he is all three of these things.