Descartes’ fourth meditation is on ‘Truth and Falsity’. His aim in this meditation is to show how and why we err.
By this stage of the meditations, we doubt everything, except for the existence of ourselves as ‘thinking things’, and the existence of a god (*Assuming Descartes’ has achieved his aims in the previous meditations*). At the end of Meditation three, and the beginning of Meditation four, Descartes’ tells us that god cannot be a deceiver, because that would suggest imperfection. Since this is not the case, we need to know how, if he is perfect, he created us to err.
Descartes aims to show that it is not god who deceives us; we are simply ‘allowed’ to be deceived. This is not an error on gods part, but in fact our fault for using the ‘gifts’ we have incorrectly.
He notes that we have infinite free will; god has not limited us in this way. However, we cannot possibly have infinite knowledge. Our capacity for error comes not from god, but from the mismatch of our level of knowledge to our infinite free will.
Descartes’ claims that we make ‘judgments’ on situations, using our free will, in spite of our lack of knowledge. This is the true nature of error. However, he also wants to give us a way of negating, or at the very least, reducing our degree of error.
He uses his notion of god, in conjunction with ‘clear and distinct’ ideas to show that if we only assent/dissent from propositions that we clearly and distinctly perceive, when we are lacking in complete knowledge, then we cannot be led astray. Where we do not have a clear and distinct idea, we must reign in our will and withhold our assent/dissent.
With assistance from clear and distinct ideas, free will, and the finite nature of our ‘nature’, Descartes places our free capacity of judgment as the sole source of error. God, much as he gave us the capacity for truth and knowledge, would not limit our free will, and so cannot stop us being wrong. God is not at fault; we are, for going beyond our knowledge even when we do not have a clear and distinct idea about the proposition in question.