Having proved the existence of God, Descartes is faced with a question. Why would God have created me such that I could be deceived? Would not this mean that God was no different from the Malicious Demon in that he could have created a world in which I wasn't deceived. But God cannot deceive me, because he is a perfect being, and to deceive one would be an imperfection. Obviously the faculty of judgement that I have is also created by God, so it too could not be imperfect, but why is it then that I can be deceived about the world as it is described in the first mediation?
I recognise that it is impossible that God should ever deceive me. For in every case of trickery or deception some imperfection is to be found; and although the ability to deceive appears to be an indication of cleverness or power, the will to deceive is undoubtedly evidence of malice or weakness, and so cannot apply to God [AT VII 53].How then to we explain that it is possible for us to be in error. We need to make a distinction between two different faculties. The faculty of Judgement or knowledge, and the faculty of the Will
When I look more closely at myself and inquire into the nature of my errors I notice that they depend on two concurrent causes, namely on the faculty of knowledge which is in me, and on the faculty of choice or freedom of the will [AT VII 56].Both faculties are perfect in themselves; otherwise I would have to say that God has created a faculty that is imperfect. The faculty of knowledge allows me to perceive ideas, and the faculty of the will allows me to assent or dissent to my judgements about these perceptions. But what does it mean for Descartes to say that these faculties are perfect? It does not mean that we are God, for it is clear that our perception of the idea of God is incomplete since we cannot understand what is represented, the objective reality, of the formal idea of God. But this impossibility is not error in itself, for it belongs to the essence of this faculty not to be able to comprehend this faculty, just as we might say that the fly is perfectly a fly even though it cannot read Descartes. We do not mean that the fly is error; rather it does not belong to the essence of the fly to be able to read Descartes.
In relation to the faculty of freedom, we can say that there are no limitations on what I can assent or dissent to. I am perfectly free to think anything that I want to think. Of course this freedom should not be confuse with any objective power. This is does not mean that I am free to fly off the top a building, even though I have no wings. Again this is not an imperfection for Descartes since it does not belong to my essence to be able to fly of the top of buildings.
If the faculty of the will, and the faculty of knowledge and judgement are perfect in themselves, then how is it possible that error can come about? It can only come about through the relation between the two faculties, rather than within the faculties themselves. The problem arise for Descartes because the faculty of the will exceeds the faculty of judgement. Thus I can assent to things, even though I do not have the correct judgement to assent to them. We might get at what Descartes has in mind here, if we think of perceptual judgement, which is the first level of deception in the first mediation. I think I see someone I recognise in the distance when it is not them, because I assent to this judgement even though I do not have the right information to do so. I then find that as I get closer that I have assented too quickly to my judgement, and in fact they are a complete stranger. It is the relation between the two faculties, therefore, that explains the possibility of error. It is not the faculties themselves that are imperfect, but how I use them
So what then is the source of my mistakes? It must be simply this: the scope of my will is wider than that of the intellect, but instead of restricting it within the same limits, I extend its use to matters that I do not understand [AT VII, 58].How then do I prevent myself falling into error? I must only assent to those things that I know, or as Descartes says the will must follow the light of reason. Thus by reason, through the intuition of the cogito, I know that I am a thinking thing, and I have assented to this compelled by reason, but there is nothing at the moment that compels me to know the difference between the mind and the body, so if I make judgement about this union I will only fall into error.
But why could not God create me with a faculty with complete knowledge, in the same way that he could have created a fly that reads Descartes. First of all, the reasons for God's creation are beyond my powers. I cannot really know why God created me in the way that he did, I can only know what I am. The reasons for existence are beyond the power of the human mind. Secondly, even if God could have created me as a more perfect being, this does not mean that with my imperfections, the universe as a whole is less perfect or would be more perfect in their absence, just as it does not make the universe less perfect because the fly cannot read Descartes.