Skepticism was founded by Arcesilaus, who was head of the New Academy from c. 268 to 241. His work was carried on by Carneades, head of the Academy in the second century B.C.E. The skeptics held that nothing could be known, and they preached epocé, which is the doctrine that all judgments, or conclusions or assessments, should be suspended. These academic skeptics posed problems, or tropes, to show that sensory knowledge is prone to error and reasoning does not necessarily result in certainty. They concluded that because we have no absolute standards for distinguishing between truth and falsehood, the best we can hope for is probable knowledge.