Nobody knows, and nobody can ever know

Absolute                       -                    Relative
Attached                      -                    Detached
'Is'                                 -                    'May be' 

Scepticism, as a doctrine of the schools, was first proclaimed by Pyrrho [...]

He is said to have maintained that there could never be any rational ground for preferring one course of action to another.

In practice, this meant that one conformed to the customs of whatever country one inhabited.

A modern disciple would go to church on Sundays and perform the correct genuflections, but without any of the religious beliefs that are supposed to inspire these actions.

'We sceptics follow in practice the way of the world, but without holding any opinion about it. We speak of the Gods as existing and offer worship to the Gods and say that they exercise providence, but in saying this we express no belief, and avoid the rashness of the dogmatizers.'

To men who, by temperament, required a gospel, it might seem unsatisfying, but like every doctrine of the Hellenistic period it recommended itself as an antidote to worry. Why trouble about the future? It is wholly uncertain. You may as well enjoy the present; 'what's to come is still unsure.'

It should be observed that Scepticism as a philosophy is not merely doubt, but what might be called dogmatic doubt.  

The man of science says 'I think it is so-and-so, but I am not sure.' The man of intellectual curiosity says 'I don't know how it is, but I hope to find out.' The philosophical Sceptic says 'nobody knows, and nobody ever can know.'

The only logic admitted by the Greeks was deductive, and all deduction had to start, like Euclid, form general principles regarded as self-evident. Timon denied the possibility of finding such principles. Everything, therefore, will have to be proved by means of something else, and all argument will be either circular or an endless chain hanging from nothing. In either case nothing can be proved.

The manner in which Arcesilaus taught would have had much to commend it, if the young men who learnt from him had been able to avoid being paralysed by it. He maintained no thesis, but would refute any thesis set up by a pupil.

Sometimes he would himself advance two contradictory propositions on successive occasions, showing how to argue convincingly in favour of either.

A pupil sufficiently vigorous to rebel might have learnt dexterity and the avoidance of fallacies; in fact, none seem to have learnt anything except cleverness and indifference to truth.

[Bertrand Russell]
History of Western Philosophy ('Cynics and Sceptics'), p.224-6, 228-9

[...] 'There are certain opinions about what is right and honourable in which we are brought up from childhood, and whose authority we respect like that of our parents.

[...] but what happens when he is confronted with the question, "What do you mean by 'honourable'?"

When he gives the answer tradition has taught him, he is refuted in argument, and when that has happened many times and on many different grounds, he is driven to think that there's no difference between honourable and disgraceful, and so on with all the other values, like right and good, that he used to revere.

What sort of respect for their authority do you think he'll feel at the end of it all?

[...] Then when he's lost any respect or feeling for his former beliefs but not yet found the truth, where is he likely to turn? Won't it be to a life which flatters his desires?

[...] And so we shall see him become a rebel instead of a conformer.

[...] there's one great precaution you can take, which is to stop their getting a taste of [philosophic discussion] too young. You must have noticed how young men, after their first taste of argument, are always contradicting people just for the fun of it; they imitate those whom they hear cross-examining each other, and themselves cross-examine other people like puppies who love to pull and tear at anyone within reach.

[...] So when they've proved a lot of people wrong and been proved wrong often themselves, they soon slip into the belief that nothing they believed before was true; with the result that they discredit themselves and the whole business of philosophy in the eyes of the world.

[...] But someone who's a bit older [...] will refuse to have anything to do with this sort of idiocy; he won't copy those who contradict just for the fun of the thing, but will be more likely to follow the lead of someone whose arguments are aimed at finding the truth. He's a more reasonable person and will get philosophy a better reputation.'

The Republic (Penguin Classics Edition), p.271-3

For the sceptic, that delicate creature, is all too easily frightened; his conscience is schooled to wince at every No, indeed even at a hard decisive Yes, and to sense something like a sting. 

Yes! and No!- that is to him contrary to morality; on the other hand, he likes his virtue to enjoy a noble continence, perhaps by saying after Montaigne 'What do I know?' Or after Socrates: 'I know that I know nothing.'Or: 'Here I do not trust myself, here no door stands open to me.' Or: 'If it did stand open, why go straight in?' Or: “What is the point of hasty hypotheses? To make no hypothesis at all could well be a part of good taste. Do you absolutely have to go straightening out what is crooked? Absolutely have to stop up every hole with oakum? Is there not plenty of time? Does time not have time? Oh you rogues, are you unable to wait? Uncertainty too has its charms, the sphinx too is a Circe, Circe too was a philosopher.' 

- Thus does a sceptic console himself; and it is true he stands in need of some consolation. For scepticism is the most spiritual expression of a certain complex physiological condition called in ordinary language nervous debility and sickliness; it arises whenever races or classes long separated from one another are decisively and suddenly crossed. 

In the new generation, which has as it were inherited varying standards and values in its blood, all is unrest, disorder, doubt, experiment; the most vital forces have a retarding effect, the virtues themselves will not let one another grow and become strong, equilibrium, centre of balance, upright certainty are lacking in body and soul. But that which becomes most profoundly sick and degenerates in such hybrids is the will: they no longer have any conception of independence of decision, of the valiant feeling of pleasure in willing - even in their dreams they doubt the 'freedom of the will’. 

Our Europe of today, the scene of a senselessly sudden attempt at radical class - and consequently race – mixture, is as a result sceptical from top to bottom, now with that agile scepticism which springs impatiently and greedily from branch to branch, now gloomy like a cloud overcharged with question-marks - and often sick to death of its will! 

Paralysis of will: where does one not find this cripple sitting today! And frequently so dressed up! How seductively dressed up! There is the loveliest false finery available for this disease; and that most of that which appears in the shop windows today as 'objectivity', 'scientificality', 'l'art pour l'art', 'pure will-less knowledge’ is merely scepticism and will-paralysis dressed up - for this diagnosis of the European sickness I am willing to go bail. 

- Sickness of will is distributed over Europe unequally: it appears most virulently and abundantly where culture has been longest, indigenous it declines according to the extent to which ‘the barbarian' still - or again - asserts his rights under the loose-fitting garment of Western culture. 

[Friedrich Nietzsche]
Beyond Good and Evil, 208

[…] without the judgement 'Benevolence is good' - that is, without re-entering the Tao - they can have no ground for promoting or stabilizing these impulses rather than any others. 

By the logic of their position they must just take their impulses as they come, from chance. And Chance here means Nature. It is from heredity, digestion, the weather, and the association of ideas, that the motives of the Conditioners will spring. Their extreme rationalism, by ‘seeing through’ all 'rational’ motives, leaves them creatures of wholly irrational behaviour. 

If you will not obey the Tao, or else commit suicide, obedience to impulse (and therefore, in the long run, to mere ‘nature') is the only course left open.

At the moment, then, of Man's victory over Nature, we find the whole human race subjected to some individual men, and those individuals subjected to that in themselves which is purely ‘natural’ - to their irrational impulses. Nature, untrammelled by values, rules the Conditioners and, through them, all humanity. 

If the fully planned and conditioned world (with its Tao a mere product of the planning) comes into existence, Nature will be troubled no more by the restive species that rose in revolt against her so many millions of years ago, will be vexed no longer by its chatter of truth and mercy and beauty and happiness. 

Ferum victorem cepit: and if the eugenics are efficient enough there will be no second revolt, but all snug beneath the Conditioners, and the Conditioners beneath her, till the moon falls or the sun grows cold.

[C.S. Lewis]
‘The Abolition of Man’, Selected Books, p.424

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