Radical scepticism has a good claim to be both the longest lasting tradition in philosophy and the consistently least popular. There’s a lot to be said for it.

By radical scepticism I mean scepticism that is in the grip of an infinite regress, like the ‘why?’ ‘…’ ‘why?’ ‘…’ ‘why?’ … of a child in the process of discovering philosophy — which is also, not coincidentally, the ‘why?’ ‘…’ ‘why?’ ‘…’ ‘why?’ … of a child discovering radical scepticism. In western philosophy this infinite regress is first discussed by Sextus Empiricus (2^nd
or 3^rd
century BC).

According to the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able to choose or to rule out anything, and we end up with suspension of judgement. In the mode deriving from infinite regress, we say that what is brought forward as a source of conviction for the matter proposed itself needs another such source, which itself needs another, and so ad infinitum, so that we have no point from which to begin to establish anything, and suspension of judgement follows.

This passage is from Sextus’s Outlines of Pyrrhonism, so named after Pyrrho (c.360-270 BC). Although Pyrrho’s works do not survive, Sextus thought of himself as a disciple of Pyrrho. For this reason, I shall use the words ‘Pyrrhonian scepticism’ or ‘radical scepticism’ to apply to any philosophy that does not escape an infinite regress.

Incidentally, some scholars have pointed out similarities between aspects of early western scepticism (as found in Sextus Empiricus) and the Buddhism that Pyrrho may have encountered when, according to Diogenes Laertius, he travelled to India in the army of Alexander the Great. However, whether or not Pyrrho and Sextus were influenced by Buddhism, it is not hard to imagine an infinite regress arising independently in the earliest stages of different philosophical traditions in unconnected parts of the world and, indeed — as we will see — this seems to have been the case.

Some scholars have pointed out similarities between aspects of early western scepticism and Buddhism. 

Contemporary philosophers, who will agree with each other on very little else, tend to agree that it is one of the defining characteristics of philosophy that it should examine its own foundations. Thus, for example, philosophers have often pointed out that it is not part of mathematics to ask ‘what is mathematics?’ and it is not part of natural science to ask ‘what is natural science?’ However, it is part of philosophy to ask ‘what is philosophy?’ For philosophy is self-reflective: it looks at its own foundations and asks ‘why?’ Radical scepticism is this …

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