In 159 BCE, ancient Greek skeptical philosopher Carneades made this statement in an attempt to refute the possibility of human beings having knowledge of anything, with knowledge defined as "belief that precludes the possibility of error." Carneades believed claiming knowledge of any sort was dogmatic.Carneades wrote:Nothing can be known, not even this!
For the skeptic, certitude of anything (even this statement) is impossible for humans to obtain. Instead, skeptics believe that humans can only assign degrees of probability to any proposition, granting higher probabilities to seemingly plausible propositions and granting lower probabilities to seemingly implausible propositions.
The principles behind skepticism are the fallibility of the human brain and the uncertain nature of reality. Science has demonstrated that the brain is capable of misinterpreting phenomena, creating hallucinations, and thinking irrationally.
Additionally, we do not even know what reality is: for all we know, we could be living in a computer simulation in which everything we can detect -- matter, energy, and other minds -- are all constructed from lines of computer code. Solipsism, the idea that nothing except one's own minds exists, could be true: after all, you could be a brain in a vat hooked up to a machine feeding you stimuli that causes your brain to react as if it were experiencing reality.
Skepticism appears the only truly rational response in the case of these considerations.
Debate question: is knowledge possible? Can anything be known with certitude? If so, how?