Sextus Empiricus was a Greek philosopher who lived in Alexandria and in Athens during the late second and early third century A.D. His best-known work, Outlines of Pyrrhonism, described a school of thought which was named after the philosopher Pyrrho of Elis (c. 365-275 B.C.). Pyrrhonism was a form of extreme skepticism which held that judgment must be suspended about whether it is possible to know true reality. Pyrrhonism asserted that suspension of judgment (epoché, a Greek term which refers to a cessation) about the true nature of reality leads to serenity and equanimity (ataraxia) about what constitutes truth or falsehood.

There are three basic approaches to epistemology, says Sextus Empiricus. The "dogmatists" assert that truth is discoverable. The "academics" deny that truth is discoverable. The "skeptics" suspend judgment and continue to search for conditions under which truth may be discovered.

Skepticism is a state of suspension of judgment. The skeptic resolves contradiction by suspending judgment. Skepticism does not affirm or deny that knowledge is possible. However, it asserts that every proposition has a co-equal opposing proposition and that every proposition is therefore susceptible to doubt.

Skepticism is different from nihilism. While nihilism asserts that nothing can be known, skepticism says that it is possible that some things may be known. Skepticism is also different from empiricism. While empiricism views experience as a source of knowledge, skepticism suspends judgment about whether knowledge is possible or impossible.

The aims of skepticism are also different from those of ethical hedonism. While the hedonist aims to enjoy pleasure and to avoid pain, the skeptic aims to achieve the tranquility of mind that is produced by being able to avoid errors of reasoning.

The skeptic questions whether the appearance of anything reflects an underlying reality. He does not affirm or deny the claim that the world reflects an underlying reality, but he questions it.

Skepticism applies the rule that things may always appear differently from the way that they actually appear. The appearance of an object may change for a perceiving subject if there is a change in the way in which the object is perceived by the subject or if there is some other change in the relation between the object and the subject. The appearance of an object may change over a period of time, or the object may become less apparent to the subject, or it may be only occasionally apparent to the subject.

Sextus does not argue that the appearances of objects are unreal or that the objects of our perceptions do not exist. Instead, he suggests that judgments concerning the true nature of objects of perception should be suspended. Although objects of perception may appear to be real, we may not be able to prove that they have a reality that is independent of our perceptions of them.

If we are skeptical about something, then we neither affirm nor deny it. The skeptic does not say that nothing can be known, but instead he asks whether there are conditions under which things may be known. However, he does not make any assertions about probability or improbability. Skepticism does not assert that any proposition is more or less probable than any other proposition.

The skeptic suspends judgment about whether there is any criterion of truth, and he says that any criterion of truth can only be judged by another criterion, which can only be judged by another criterion, and so on ad infinitum. Thus, the attempt to demonstrate any criterion of truth leads to a form of circular reasoning and cannot arrive at any conclusion.

To suspend judgment about the truth or falsehood of a logical proposition may be to suggest that there is some uncertainty about its truth or falsehood. Sextus does not say that there must be a sufficient degree of uncertainty about the truth or falsehood of a proposition in order to justify a suspension of judgment, but he says that no proposition can be proved with complete certainty.

Skepticism does not make any assertions about reality or unreality. It does not affirm or deny that objects of perception exist in reality or that they do not exist in reality. The sceptic suspends judgment about the reality of physical laws, about the reality of matter, and about the reality of time and space.

Skepticism also leads to a suspension of judgement about whether there are any causal relations between events or phenomena. It neither affirms nor denies that any event is the cause of another event, and it suspends judgment about the existence of causality as an explanation for why things appear in the way in which they actually appear.

The skeptic questions whether anything can be proved, because any proof must be proved by another proof, which must be proved by another proof, and so on ad infinitum. Thus, for the skeptic, there is no self-evident proof of anything.

Ethical skepticism suspends judgment about how good and bad should be defined, and it suspends judgment about whether anything is instrinsically good or evil. It asserts that any moral criterion of what is right or wrong may be susceptible to doubt.

The skeptic also suspends judgment about the existence or non-existence of God. For the skeptic, there is no self-evident proof of the existence of God. While an agnostic may say that it is impossible for us to know whether or not God exists, the skeptic says that we should suspend judgment about whether or not God exists.

Sextus Empiricus' skepticism attempts to avoid error by suspending judgment about questions which cannot be resolved with certainty. This suspension of judgment applies not only to metaphysical questions (such as the origin of the universe or the nature of ultimate reality), but also to ethical, aesthetic, and logical questions. Skepticism refuses to accept any metaphysical, ethical, aesthetic, or logical propositions which have not been subjected to careful scrutiny.

This suspension of judgment is not, in any absolute sense, proposed as a formula for finding truth. Skepticism is more a method of avoiding error than a method of finding truth. The skeptic avoids belief or disbelief in anything, because belief or disbelief may produce conflict instead of equanimity concerning the nature of truth or falsehood.

A weakness of this viewpoint, however, is that this attitude of systematic doubt inevitably leaves skepticism unable to prove its own conclusions. Skepticism must be careful to avoid self-contradiction, because the rule that everything is susceptible to doubt may itself be susceptible to doubt.