What would be an epistemologist's reason for doubting all knowledge and reasoning?

For ex. how can 2 + 2 = 4 not be true? When you add a certain amount together you get both the amounts combined. It's not as if numbers can deceive us because numbers are not entities or physical objects they are just a way of counting and measuring etc. I could argue that something physical can be doubted for ex. the sky could be red instead of blue and our eyes could have been playing tricks on us the whole time. The sky is more likely just how we see it but there could still be a possibility that it's not. According to Descartes and others that would agree with him, anything that involves the senses could be doubted and the only truth lies in thinking ("I think therefore I am") Alot of philosophers have pointed out assumptions that he made in order to get from "I think" to "I am" and I'm not going to bother listing all of them. One assumption, a couple of people interested in philosophy, had pointed out was that he assumed logic to be true. It is using logic to come to the conclusion that in order to think you have to exist but the way I understand what Descarte's meant was that thoughts exist and for him thoughts and self were equal. Meaning he could be his own thought process and nothing more. Does it take logic to realise that you are thinking, or that you are aware of the world around you? If it does I would have to say that we can doubt our own logic only to a certain extent because if the logic in realising your own thoughts is wrong then the only other possibility I can think of is that there are not thoughts or awareness of any kind. It's like saying that there is no such thing as experiencing anything or thinking in any way.