Skepticism. Major Thinkers include Pyrrho (361-270 BCE), Arcesilaus (316-241 BCE), and Carneades (214-129 BCE). Key Concepts. Greatest good comes from peace of mind (ataraxia) which can achieved by accepting our limits of understanding. There can be no certainty in human knowledge. Skeptics are involved in a continual inquiry for the truth. Probability of correct morality is enough for acting in a moral fashion; existing ethical and legal systems are reliable guides.