When we talk about Avicenna and Averroes, we must see how there is a progress, how Avicenna's understanding leads to Averroes' conception. Materialism progresses : for Avicenna, God was justified by Metaphysics, and Metaphysics were justifying Physics. But in Averroes' thought, God is justified by Physics, not by Metaphysics.
This is indeed an important move. Avicenna considers therefore that all causes must have a source, a cause of the cause. So, God “must” exist, as the primary cause, which is uncaused. This is a conception conform to Aristotle's lessons.
Averroes, on his side, goes further. His god is “passive”, like in Spinoza's conception afterwards. So, as he is eternal – like the the world – he exists.
Avicenna promoted a pre-materialist conception using an active God. Averroes tried to build materialism through an eternal passive God.
In doing this, Averroes purges Aristotelianism from the elements coming not from Aristotle, but from Plato and that were combined to it in an erroneous manner at the beginning of the first millenium.
Averroes attacks directly the concept of a God sending a sort of energy, “emanations”: he doesn't believe that God is a unity, a “one” which emanates plurality, from which, at the end, there is our world.
Averroes separates radically God from the world – and in doing this, he puts God in the world, as a mere motor. It is a religious system masking a materialist system. Here is his criticism from the Falsafa before him:
“About this statement that out of the one only one proceeds —all ancient philosophers were agreed.
When they investigated the first principle of the world in a dialectical way they mistook this investigation, however, for a real demonstration and they all came to the conclusion that the first principle is one and the same for every thing and that from the one only one can proceed...
But when the philosophers of our religion, like Fārābi and Avicenna [the two others main figures of the Arabic-Persian Falsafa], had once conceded to their opponents that the agent in the Divine world is like the agent in the empirical and that from the one agent there can arise but one object (and according to all the First was absolutely simple unity), it became difficult for them to explain how plurality could arise from it....
They declared that from the First, who is a simple existent, the mover of the highest sphere proceeds, and from this mover, since it is of a composite nature, as it thinks both itself and the First, the highest sphere, and the mover of the second sphere, the sphere under the highest, can arise.
This, however, is a mistake, according to philosophical teaching, for thinker and thought are one identical thing...
This does not affect Aristotle's theory, for the individual agent in the empirical world, from which there can only proceed one single act, can only in an equivocal way be compared to the first agent.
For the first agent in the divine world is an absolute agent, while the agent in the empirical world is a relative agent, and from the absolute agent only an absolute act which has no special individual object can proceed.” (Averroes, Tahafut al-tahafut)
So, there is only one God, as the concept of God can only be one, as one can only be one and give one. So logically God does not create the world, he is the world for Spinoza, and in between historically, he is the motor of the world.
Spinoza suppressed the existence of God as separated of the world: in his thought God is Nature, not a motor. But in doing this he suppressed the movement, and Hegel theorized this missing element.
But interestingly, we find still movement in Averroes' thought, as God is a motor that permits forms to exist through matter. For example, a human is a form that forms matter as human; it is its goal.
The realization of this goal is good: each form has a goal, and its realization is good; the motor is happy from this realization, as he is final in itself and so the supreme goodness. This is Aristotle's conception.
Averroes still uses this understanding, which is idealistic because of the cause-consequence principle.
So, how does he see “movement”? As he rejects matter existing for itself – matter is nothing if a form does not come to “form” it – then, he must justify it somewhere else.
And as his God is not “active” he can not, like Avicenna, put in God neither.
So, he tried to find another way; here is what he says:
“Non-existence is the opposite of existence, and each of the two is succeeded by the other, and when the non-existence of a thing disappears it is followed by its non-existence.
As non-existence by itself cannot change into existence, and existence by itself cannot change into non-existence, there must be a third entity which is the recipient (al-qabil) for both of them, and that is what is described by “possibility” (imkan) and “becoming” (takawwun) and “change (intiqal) from the quality of non-existence to the quality of existence”.
For non-existence itself is not described by “becoming” or “change” nor is the thing that has become actual described in this way, for what becomes loses the quality of becoming, change, and possibility when it has become actual.
Therefore there must be necessarily something that can be described by “becoming” and “change” and “transition from non-existence to existence”, as happens in the passage of opposites into opposites; that is to say, there must be a stratum for them in which they can interchange – with this one difference, however, that this stratum exists in the interchange of all the accidents in actuality, whereas in the substance it exists in potentiality.” (Averroes, Tahafut al-tahafut)
The key of the transformation of the opposites searched by Averroes is the dialectic of matter, the “something” described as “becoming” and “change” and “transition from non-existence to existence” is the universe as an onion according to dialectical materialism.