In this extremely unlovely book, Sextus Empiricus (c. 200 CE) summarizes the main points of skeptical philosophy following the tradition of the semi-legendary philosopher Pyrrho (c. 300 BCE). Pyrrhic skepticism directs us to question dogmatic or uncertain statements, as one would tend to think of skepticism in the time of Descartes. But it was also an explicitly therapeutic discipline aimed at generating a sense of detachment or equipoise in which the mind would stand in a state of suspension or non-commitment to beliefs. This was said to relieve the practitioner from unnecessary suffering. This work by Sextus Empiricus provides a terse and extremely dry collection of typical arguments the school would employ in bringing about that state of uncertainty.
If this sounds similar to the Yogasutra of Patanjali or the Buddhist philosophers of India, this is not a coincidence. According to Diogenes Laertius, Pyrrho traveled to India in order to study with the Gymnosophists, the Greek word he used to describe various yogic schools. The philosophical and rhetorical agreements between the Madhyamaka school of Nagarjuna and Pyrrhic skepticism are countless, ranging from the stated goals of the tradition to using extremely similar examples, such as both traditions using the case of phenomena appearing yellow to a person suffering from jaundice as a stock example for why the senses are unreliable.
The views of the skeptical schools of Pyrrho and Timon were summarized by a certain Aristocles in this manner:
"[W]e should not trust [sense perceptions, as they are unreliable], but should be without opinions and without inclinations and without wavering, saying about each single thing that it no more is than is not, or both is and is not, or neither is nor is not. Timon says that the result for those who are so disposed will be first speechlessness [perhaps resembling the "noble silence" that fell upon the Buddha after his final illumination], but then freedom from worry; and Aenesidemus says pleasure."
We are first struck by the similarity to the Madhyamaka ideal of "freedom from views," and then by the rejection of four possible modalities of being: is, is not, both, and neither. This fourfold negation is referred to by Buddhist scholars as the tetralemma, and is universally acknowledged to be the definitive position associated with the Buddhist Madhymaka school. I have never myself seen it formulated in a non-Indian tradition from the ancient world.
Here is a short excerpt from Outlines of Skepticism:
"If a thing moves, it moves either in the place where it is or in that where it is not. but it does not move in the place where it is, for if it is in it, it remains in it; nor yet does it move in the place where it is not; for where a thing is not, there it can neither effect nor suffer anything. Therefore nothing moves."
Compare to this argument from Nagarjuna's Mulamadhymakakarika:
Now, where one has gone one does not go.
Where one has not yet gone one does not go.
Apart from where one has gone and where one has not gone,
That over which one goes cannot be conceived.
This work is therefore of extraordinary interest to the comparativist, and I think one would have to be obtuse to fail to recognize the obvious connection here between Indian and Greek philosophy. What form it took, we cannot say. According to our tentative dating, Pyrrho was centuries earlier than Nagarjuna, but then Nagarjuna is often held to have compiled existing ideas in his Fundamental Wisdom.
The arguments of Sextus Empiricus also exerted a deep influence on modern philosophy, notably on Kant and Hegel, both of whom are known to have been enthusiastic readers of Pyrrhic skepticism. Kant taught Sextus Empricus for thirty years as part of his course on logic, and the basic structure of his use of antimonies of pure reason reflects its obvious influence. Hegel's criticism of dogmatic philosophy, as has been pointed out by his biographer Klaus Vieweg, owes a great deal to his enthusiastic use of Sextus Empiricus's so-called "five modes," which are presented as a general set of arguments for refuting any position.
The five modes are often reduced to a subset of three, which are referred to as the Trilemma of Agrippa. In this form, the argument runs as follows:
If I assert a proposition P, for which I provide some number of reasons or arguments, then there are either a finite number of reasons, or an infinite number of reasons; in the latter case, the argument may be eliminated by reductio ad infinitum.
If there are a finite number of reasons for the argument (P because of Q, Q because of R, and so forth), either the successive justifications eventually lead back to the original argument, or they do not; in the former case, it is a circular reasoning. And in the remaining case, in which a finite number of arguments terminates on a proposition not identical to the initial proposition, that final proposition must itself be unestablished, and thus dogmatic.
All arguments therefore ultimately resolve through chains of reasoning either back to themselves, or to an article of faith, or they do not resolve at all.
Vieweg persuasively links this argument to Hegel's critique of Schelling in the Differenzschrift as containing covert dogmatic premises, which are themselves undemonstrated.
Again, I have to emphasize that Outlines is not a beautiful book. It is not readable like a Platonic dialog, but it may be mined for extremely valuable material.