Al-Ghazali (c. 1057–1111) 的 Deliverance from Error 是从年轻时就知道审视错误开始讲起的：
In the bloom of my youth and the prime of my life, from the time I reached puberty before I was twenty until now, when I am over fifty, I have constantly been diving daringly into the depths of this profound sea and wading into its deep water like a bold man, not like a cautious coward. I would penetrate far into every murky mystery, pounce upon every problem, and dash into every mazy difficulty. I would scrutinize the creed of every sect and seek to lay bare the secrets of each faction’s teaching with the aim of discriminating between the proponent of truth and the advocate of error, and between the faithful follower of tradition and the heterodox innovator.
I realized, then, that whatever I did not know in this way and was not certain of with this kind of certainty was unreliable and unsure knowledge, and that every knowledge unaccompanied by safety from error is not sure and certain knowledge.
Descartes 的 Meditations on First Philosophy (1641) 则是先意识到自己年轻时的错误：
Several years have now passed since I first realized how numerous were the false opinions that in my youth I had taken to be true, and thus how doubtful were all those that I had subsequently built upon them. And thus I realized that once in my life I had to raze everything to the ground and beginagain from the original foundations, if I wanted to establish anything firm and lasting in the sciences.
For this reason, it will suffice for the rejection of all of these opinions, if I find in each of them some reason for doubt.
接着，两人都开始考察 sense perception。Al-Ghazali 说：
I then scrutinized all my cognitions and found myself devoid of any knowledge answering the previous description except in the case of sense-data and the selfevident truths.
Surely whatever I had admitted until now as most true I received either from the senses or through the senses.
两人（不？）约而同都拒绝了 sense perception。Al-Ghazali 说：
With great earnestness, therefore, I began to reflect on my sense-data to see if I could make myself doubt them . . . “Whence comes your reliance on sense-data? The strongest of the senses is the sense of sight. Now this looks at a shadow and sees it standing still and motionless and judges that motion must be denied. Then, due to experience and observation, an hour later it knows that the shadow is moving, and that it did not move in a sudden spurt, but so gradually and imperceptibly that it was never completely at rest. Sight also looks at a star and sees it as something small, the size of a dinar: then geometrical proofs demonstrate that it surpasses the earth in size. In the case of this and of similar instances of sense-data the sense-judge makes its judgments, but the reason-judge refutes it and repeatedly gives it the lie in an incontrovertible fashion.
However, I have noticed that the senses are sometimes deceptive; and it is a mark of prudence never to place our complete trust in those who have deceived us even once.
Al-Ghazali 顺藤摸瓜继续怀疑了 reason（虽然随后会说 reason 是不证自明的）：
Then sense-data spoke up: “What assurance have you that your reliance on rational data is not like your reliance on sense-data? Indeed, you used to have confidence in me. Then the reason-judge came along and gave me the lie. But were it not for the reason-judge, you would still accept me as true. So there may be, beyond the perception of reason, another judge. And if the latter revealed itself, it would give the lie to the judgments of reason, just as the reason-judge revealed itself and gave the lie to the judgments of sense. The mere fact of the nonappearance of that further perception does not prove the impossibility of its existence.”
随后，两人都开始讲做梦。Al-Ghazali 用做梦来说明 reason 不可信任：
For a brief space my soul hesitated about the answer to that objection, and sense-data reinforced their difficulty by an appeal to dreaming, saying: “Don’t you see that when you are asleep you believe certain things and imagine certain circumstances and believe they are fixed and lasting and entertain no doubts about that being their status? Then you wake up and know that all your imaginings and beliefs were groundless and unsubstantial. So while everything you believe through sensation or intellection in your waking state may be true in relation to that state, what assurance have you that you may not suddenly experience a state which would have the same relation to your waking state as the latter has to your dreaming, and your waking state would be dreaming in relation to that new and further state? If you found yourself in such a state, you would be sure that all your rational beliefs were unsubstantial fancies.
Descartes 则认为 mathematical truths 即使在梦中也是不可怀疑的：
Let us assume then, for the sake of argument, that we are dreaming and that such particulars as these are not true: that we are opening our eyes, moving our head, and extending our hands. Perhaps we do not even have such hands, or any such body at all . . .
. . . For whether I am awake or asleep, 2 plus 3 make 5, and a square does not
have more than 4 sides. It does not seem possible that such obvious truths
should be subject to the suspicion of being false.
When these thoughts occurred to me they penetrated my soul, and so I tried to deal with that objection. However, my effort was unsuccessful, since the objection could be refuted only by proof. But the only way to put together a proof was to combine primary cognitions. So if, as in my case, these were inadmissible, it was impossible to construct the proof.
那怎么办呢？当然是 God 辣！Al-Ghazali：
This malady was mysterious and it lasted for nearly two months. During that time I was a skeptic in fact, but not in utterance and doctrine. At length God Most High cured me of that sickness. My soul regained its health and equilibrium and once again I accepted the self-evident data of reason and relied on them with safety and certainty. But that was not achieved by constructing a proof or putting together an argument. On the contrary, it was the effect of a light which God Most High cast into my breast. And that light is the key to most knowledge.
Reason 在 al-Ghazali 看来是不证自明的（因为 God），并且是不可能证明的。Descartes 则把这个证明写出来，引发了循环论证的忧虑。