The Three Great Sceptics were, in chronological order, Karl Marx (5 May 1818 – 14 March 1883), Friedrich Nietzsche (15 October, 1844 – 25 August, 1900) and Sigmund Freud (6 May 1856 – 23 September 1939). Each of these theorists in some way or another could be said to have engaged in some form of Epistemology, particularly subjective epistemology, which argues that knowledge is not necessarily possible to attain in the first place, in the case of Nietzsche to the extent where he would at times be accused of solipsism, the ideal that the only certainty is that one's own mind exists.
The Marxist and Nietzschean views can logically be said to largely clash with each other based on the grander scopes of their beliefs, Marxists believing that particular levels of democracy are the means by which to progress towards a Socialist society and thus the means by which humanity is to further develop, however it was believed that the style of democracy necessary for the restructuring of society was not that present within that time as it was the democracy governed by the rich and powerful, thus the proletariat never having the true opportunity for freedom. Marx (and Engels) put forth that the only way towards a true democracy would be for first a revolution to come and topple the existing governing by the bourgeoisie class and then for a new democratic order to arise.
Conversely Nietzsche was radically against the ideals of democracy, believing that only the strong and intelligent should lead and that this would be the true means by which the human race could evolve and develop. His belief was that the human race in itself must be destroyed as a means by which to lead the way to the next evolutionary steps of the individuals deemed worthy and tenacious enough to progress onto the next stage of existence which he termed the Ubermensch or overman. He advocated indulgence instead of abstinence as a means by which to truly experience life and progress to the next stage of existence. Of all the Sceptics, he could be said to be the most detached from any ideals of religion or morality as he was purely about the self and feeding the self. His ideals were primarily based within the ID, though it is apparent he did not believe in subconscious desires and merely conscious ones that were suppressed by misconceived preconceptions of enforced morality dictated by religion - "I condemn Christianity", he said this because he believed that Christianity, along with most other organised religion, had forced an intolerable doctrine upon the people that must be removed - "We must do philosophy with sledgehammers", with this Nietzsche was saying that everything should be smashed to pieces and built up from scratch, opening the possibilities to advance within humanity into the creation of the ubermensch.
This said, both are at odds with the Freudian theories owing to a lack of anthropological basis or a form of political framework to explain why particular individuals might experience the psychological ailments they suffer from. Whereas Nietzsche and Marx extensively researched various cultures and ritualistic systems and thus were fully aware that certain practices such as cannibalism and child marriage, whilst seen as abhorrent within our own culture, could be viewed as entirely normal within another culture. Therefore to simply categorise all under one specific set of rules to both Nietzscheans and Marxists would seem unreasonable and lacking in any truly empirical basis.
It is somewhat ironic that Sigmund Freud is considered one of the great sceptics, given that a huge amount of his theories are widely disregarded owing to a lack of sufficient empirical data.
All this said, each are regarded as Great Sceptics for the fact that their theories constantly challenged a vast majority of established ideals.
Marx's ideals of the ruling law being that of the ruling class and that the lower classes should rise up against the bourgeoisie greatly rivals a long-believed ideal put forth by Onosander that people will not follow one who is not of noble birth.
Schopenhauer's 'Will to Live', written a generation before Nietzsche, put forth the ideal that everything in the universe was striving towards the desire to exist and procreate and therefore above all else this was what was prioritised. Nietzsche's 'Will to Power' contested this ideal by putting forth the suggestion that in truth everything is striving for power. According to Nietzsche this can be seen in both the natural and man-made worlds in the act of warfare, mating rituals and the like, in which the individual will risk their life for the sake of honour, glory and power. Simply living is not enough for them, for quite often they will die for the sake of gaining immortality in the minds of others. The will to power also stands as an argument against the ideals of Utilitarianism and Platonism, desires for goodness and unity, because these ideals do not explain why humans have often such volatile and unpredictable natures.
Freud is probably regarded as the most radical and pessimistic of the Great Sceptics as he believed that humanity on the whole had no point to which they would advance towards and that they would simply constantly strive for sheer self-destruction and the destruction of others. This contests all theories placed forth, including those of Nietzsche and Marx, who contrarily believed that humanity could advance if it learned how specifically to do so. Strangely enough, despite being a self-proclaimed atheist, his theories on the status of being were steeped in the ideals of abstinence and self-control, reinforcing the ego through his techniques so as to better control the Id.
In all cases there would have been great contesting of the supposed universal truths in three sentences presented by Frege:
1 - That the Morning Star is the same as the Evening Star
2 - That the present King of France is not bald
3 - That there was no one on the road
The significance of these sentences is that all of them can be argued against being universal truths as follows:
1 - You would require prior knowledge and understanding of certain mechanics to know that this is actually fact
2 - This entirely depends upon the context of the time and each individuals knowledge of France at that time
3 - Although at any one time on any given road there may be no one on it, that does not mean that on another road there will not be someone. It also depends upon what you consider to be the road in question, whether it is just one particular stretch or the entirety of all roads connected up.
In each case the evidence for these statements can be disputed and as such can not be considered universally true.
