a) Presuppositional apologetics leads to the conclusion that unbelievers don't know anything
Actually nothing could be further from the truth. A presuppositionalist insists that unbelievers know a lot. Sometimes they even know more than a believers knows. The problem is not whether an unbeliever knows something or not, but whether his basic assumptions, his “presuppositions”, make sense of what they claim to know or not.
Greg Bahnsen put it this way: “Because fallen man cannot escape the knowledge of God, he is still able to acquire knowledge through his contact with general revelation in the world; as a creature of God he retains an irradicable knowledge of his Creator, knowledge of himself always bringing knowledge of his God as well … the unbeliever … can and does use his mind to know things; he is able to do so despite of what he thinks about his epistemological situation.” (Presuppositional Apologetics, Greg Bahnsen, pp. 16-17).
It is in fact the knowledge that the unbeliever has that shows his inconsistency. We are not asserting in any way that unbelievers don’t know things. On the contrary, we are insisting they know quite a lot, and at the same time we are also insisting the knowledge they have doesn’t conform with their own presupposition but rather with the Christian’s. If they were consistent with their presuppositions, they wouldn’t be able to justify any knowledge whatsoever whether we’re talking about chicken or the hereafter. So the knowledge they have proves that they already know the God they are trying to deny, because such knowledge is only possible within a Christian worldview. What unbelievers know, they know not because of their presuppositions, but in spite of them.
By way of an example, an unbeliever may declare: There is no such thing as “immaterial entities”. Yet they insist on using the laws of logic, which are not material. Or they may say: Everything is subjective and there is no such thing as objective morality. How then can they condemn Hitler for the Holocaust, for example? If there really were no objective morality, then whatever Hitler did could not be condemned or condoned by any one; it would be nothing more than what McDonalds does to potatoes daily - a bag of biological stuff did something to another bag of biological stuff and nothing more to it … simply put, it would be “Hitler did what Hitler did”.