I know nothing except the fact of my own ignorance. --Socrates

There are some truths that are completely unknowable to people. This is the credo of the Academic Skeptic. This branch of skepticism comes from the Platonic Academy which gradually adopted this position after Plato's death.

Contrary to Pyrrhoean skeptics, the Academic skeptics deny that knowledge is possible, at times even dogmaticly. This opens them up to the classic argument against Academic Skepticism:

Do you know that knowledge is impossible? Either you cannot know that, or some knowledge is possible.

This retort and its variations are the single most common objection to skepticism. However, it applies to very few of the Academic Skeptics, and to none of the other branches of skepticism. Still, many philosophers believe that skepticism is self-refuting, as Descartes said, that doubting cannot doubt itself.

Academic skeptics have been aware of this retort for centuries and are certainly not foolish enough to engage it without proper defense. Some followed Cicero and held that no knowledge is possible except for the one truth that no other knowledge is possible. This exception is hard to justify to some, but at least seems to free it of contradiction. Others believe that quite a bit of knowledge is attainable, but the unknowable is also very large and includes all the truly important questions, such as the nature of humanity, the soul, the gods, change, reality, free will, immortality, and the whys of ethics.

Protagoras declares that one can take either side on any question and debate it with equal success - even on this very question, whether every subject can be debated from either point of view. --Seneca

According to Cicero, at least four ancient philosophers took the position that nothing can be known except the proposition that nothing (else) could be known: Socrates, Varro, Antipater, and Cicero himself. Others took the position that nothing could be known including the proposition that nothing could be known: Arcesilas, Carneades, and Chian Metrodorus.

Academic skeptics limited themselves to denying the possibility, not of knowledge per se, but of certainty. We might chance upon the true explanation of something important, but we can never know it because it would be uncertain and in now way discernible from the competing false explanations.