One
explanation from Stanford Encyclopedia of Philosophy
A way to motivate Academic Skepticism and to clearly distinguish it from ordinary incredulity is to trace the way in which Descartes expanded the realm of what was doubtful (and hence not worthy of assent) in the “First Meditation.”[7] Descartes begins by noting that the senses have deceived him on some occasions and, in the voice of his skeptical interlocutor, he conjectures that it is never prudent to trust what occasionally misleads. So, we don't have “certain” knowledge of the external world based upon the testimony of our senses. However, in the voice of the non-skeptical interlocutor, he replies that even though the senses have misled him, he can neutralize that purported basis for doubt by pointing out that we are able to determine when our senses are not trustworthy. Thus, this is a case of ordinary incredulity because he appeals to some knowledge of the world gained through our senses to neutralize this basis for doubt. For example, in looking at a straight stick in water, even though it appears bent, we know not to accept the testimony of our senses at face value. We can neutralize the potentially knowledge-robbing proposition that my senses have deceived me on some occasions by adding to it another proposition to which we assent, namely, I can distinguish between the occasions when my senses are trustworthy and those when they are not. Some propositions in the EI-type (propositions about the external world) can be used to rebuff the grounds for ordinary incredulity. Thus, no basis for (philosophical) Academic Skepticism has been located.
Descartes next considers dreaming. What if he were dreaming at that very moment? Would he still have some knowledge of the external world? Yes; because in dreams and in waking life there are some common general features. So, if he were dreaming, he would not know in particular what is going on about him at that moment, but that does not imply that he fails to have any knowledge at that moment. For example, he still could know that there are hands. More importantly, even more simple things about nature “in general” are not thereby made doubtful. We have not found any reason for doubting that there are material objects or that they have a spatial location, or are in motion or at rest, or can exist for a long or short period of time. Again, no basis for Academic Skepticism has been established.
But then Descartes thinks of a grounds for doubt for which he says he “certainly has no reply.” He puts it this way:
… In whatever way [it is supposed] that I have arrived at the state of being that I have reached — whether [it is attributed] to fate or to accident, or [made] out that it is by a continual succession of antecedents, or by some other method — since to err and deceive oneself is a defect, it is clear that the greater will be the probability of my being so imperfect as to deceive myself ever, as is the Author of my being to whom [is assigned] my origin the less powerful. (Meditations, 147)
In other words, at this point in the Meditations, since he lacks an argument for the claim that whatever is causally responsible for his “state of being” is capable of making his being such that to err would not be natural for it, assenting to propositions arrived at by his “state of being” is not legitimate. Thus, Descartes believes that he has located a basis for doubting all of his supposed former knowledge of the external world that cannot be repulsed by locating another such proposition to which he is entitled. He has found a proposition that, if true, would (by itself) defeat the justification he has for his assenting to propositions about the external world and which is such that (1) he does not (at least at this point in the Meditations) have a way to reject it and such that (2) he has no way to neutralize its effect. Thus, a basis for philosophical skepticism has been found because an entire class of EI-type propositions — propositions that his “nature” has led him to assent to — is now thrown into doubt because he cannot use one member of the class to reject or neutralize the basis for doubting another member of the class.
Descartes apparently thinks that something is worthy of assent only if it is immune to genuine doubt. It appears that he thinks that something, d, is a grounds for genuine doubt of p for S iff:
1. d added to S's beliefs makes assent to p no longer adequately justified;
2. S is not justified in denying d;[8]
3. S has no way to neutralize d.[9]
The final step is to say that some proposition is not worthy of assent if there are genuine grounds for doubting it. Indeed, Descartes grants that even after d is located, p might still be more reasonable to believe than to deny (Meditations, 148). The point is that the pro-attitude should not rise to the level required for knowledge because there is a genuine ground for doubt. Thus, a crucial feature of the Cartesian-style argument for Academic Skepticism is that it employs a very stringent requirement on the type of evidence required for knowledge. It must make a proposition immune to genuine doubt.