Descartes uses intellect to prove that God is the guarantee of the truth, and disproving the evil demon. But if this is true, why when i make purely intellectual reasoning, (because i can make a mistake with an exterior knoledge, like not knowing how to spell), but just using my mind i still make mistakes. Does that refute Descartes philosophy?
Descartes himself considers the question how we can have false beliefs, given our having been created by a perfect God in the Fourth Meditation. The ultimate answer, Descartes thinks, is because God has given us a power to know the true from the false, but that power is limited, not infinite. Therefore, when we use our intellects to think about things beyond the scope God designed them to work for, we fall into error. In other words, it isn't God's fault we have false beliefs. It is our fault for using our minds improperly.
And no doubt respecting this matter could remain, if it were not that the consequence would seem to follow that I can thus never be deceived; for if I hold all that I possess from God, and if He has not placed in me the capacity for error, it seems as though I could never fall into error. And it is true that when I think only of God [and direct my mind wholly to Him],18 I discover [in myself] no cause of error, or falsity; yet directly afterwards, when recurring to myself, experience shows me that I am nevertheless subject to an infinitude of errors, as to which, when we come to investigate them more closely, I notice that not only is there a real and positive idea of God or of a Being of supreme perfection present to my mind, but also, so to speak, a certain negative idea of nothing, that is, of that which is infinitely removed from any kind of perfection; and that I am in a sense something intermediate between God and nought, i.e. placed in such a manner between the supreme Being and non-being, that there is in truth nothing in me that can lead to error in so far as a sovereign Being has formed me; but that, as I in some degree participate likewise in nought or in non-being, i.e. in so far as I am not myself the supreme Being, and as I find myself subject to an infinitude of imperfections, I ought not to be astonished if I should fall into error. Thus do I recognise that error, in so far as it is such, is not a real thing depending on God, but simply a defect; and therefore, in order to fall into it, that I have no need to possess a special faculty given me by God for this very purpose, but that I fall into error from the fact that the power given me by God for the purpose of distinguishing truth from error is not infinite. [emphasis mine.]
You'll find, if you continue reading on after this passage how Descartes thinks the will enters into this picture.
Along with his certainty of God's existence, Descartes also recognized his own finitude and inability to understand everything:
"And this does not cease to be true although I do not comprehend the infinite, or though in God there is an infinitude of things which I cannot comprehend, nor possibly even reach in any way by thought; for it is of the nature of the infinite that my nature, which is finite and limited, should not comprehend it; and it is sufficient that I should understand this, and that I should judge that all things which I clearly perceive and in which I know that there is some perfection, and possibly likewise an infinitude of properties of which I am ignorant, are in God formally or eminently, so that the idea which I have of Him may become the most true, most clear, and most distinct of all the ideas that are in my mind." (3rd Meditation)
Thus, there is nothing in his writing to suggest that God guarantees that we should never make any mistakes.