This book explains a very thoughtful, rigorously worked out consideration of the following perplexing observation, which is one I think we all share: "I do not seem to know anything for certain." By having so carefully considered this issue, I believe that this ancient book represents a coherent and complete answer to the predicaments that modern skeptics so worry and strain themselves over; for example, it achieves Sartre's own goal, which was to "work out a coherent atheism," and did so 2000 years before Sartre was born.
The Outlines, like the other extant works of Sextus Empiricus, is largely a recording of teachings attributed to a Greek philosopher of the 4th c. B.C. named Pyrrho of Elis. Pyrrho is a shadowy figure and himself left no extant writings, but is believed by longstanding rumor (preserved most quote-ably by the Roman historian Diogenes Laertius) to have been influenced by Buddhism during his travels with Alexander the Great to India.
Pyrrho's thought influenced middle and later phases of Plato's Academy and flourished there for some centuries, where it was intensely worked and re-worked. Indeed, Pyrrho's thought ultimately exerted such great influence in classical civilization that his name became synonomous with the modern technical meaning of the word "skepticism" (in fact, the title of this work, which in Greek is "Pyrrhoniae Hypothesi," is sometimes translated as "Outlines of Skepticism").
Ancient skepticism fell into obscurity following the fall of Rome and languished in obscurity for nearly a millennium. Fortunatley, however, the works of Sextus were rediscovered during the Italian Renaissance and from there enjoyed wide attention in Europe for some centuries, impacting the works of such notable figures as Montaigne and Walter Raleigh.
Nevertheless, ancient skepticism again fell out of academic view in more recent times. This is peculiar and unfortunate; this body of thought was no less influential than Platonic, Aristotelian, and other classical movements now effectively canonized in Western culture and was kept well in the forefront of academic thought for many centuries, but is now largely a curiosity seriously studied only by specialist philosophers and classics scholars.
What is most interesting to me about ancient skepticism is that I think everything that could possibly be said by modern doubters -- the phenomenologists, the existentialists, the mass of usually unthinking and poorly educated oafs who call themselves postmodernists -- was already said by the ancients. Indeed, the absolutely key points that a doubter must make in order to render his doubts even coherent all appear in the Outlines, in my opinion, and I see nothing in the supposedly radical works of modern day doubters that is really more radical than what is contained in Sextus.
Finally, there is no better introduction to ancient skepticism than the Outlines. Sextus is unbelievably straightforward and easy to understand, especially if you have any experience reading other works of skepticism.
Personally, I think the Barnes & Annas translation, available in an in-print Cambridge University edition, is better because it is better suited to modern readers and is copiously annotated. However, this or any other edition will do for a non-specialist looking for an understanding ancient skepticism.
This work is a classic; it largely is a recording of the otherwise unavailable teachings attributed to a Greek philosopher of the 4th c. BC named Pyrrho of Elis. The reason I wrote this, however, is that the "synopsis" presented by Amazon summarizing the book is seriously faulty. First of all, I can't imagine why it says that this book considers "th[e] theory as set forth" by Sextus that "certain knowledge is possible--that the physical world and ideas formulated about it could be given solid foundations unaffected by the varieties of mere opinion." Sextus, and Pyrrho, the man of whose teachings Sextus is mostly just a scribe, believed precisely the opposite. I think the synopsis may just have been poorly written, but in any case it gives an incorrect impression.
Second, it is deeply false to call Sextus the "founder of the 'skeptic' school of thought." Though no one is sure when Sextus lived, it was several centuries after Pyrrho, and even Pyrrho couldn't be called the founder of Skepticism. His teacher Anaxarchus taught a reasoned and systematized series of arguments that explained his epistemological doubts. Moreover, Plato's work "Theatetus" also sets forth detailed epistemological skepticism, and that work predates even the lifetime of Anaxarchus. "[S]keptic school[s] of thought" had flourished for centuries in Greece and Rome before Sextus was even born.
The synopsis is misleading and inaccurate.