Knowledge of The External World - Scepticism

• We can know x exists if we have the experience of it and we can infer and ponder upon what we have experienced and we can assume x has actually happened.
• The central problem with empiricism is that it doesn’t transcend our own senses, we cannot experience what other people do and we cannot check whether what we experience is physically there. We are sense-dependent. We do not know if these mental impressions are the truth.
• A sceptic would claim that our senses sometimes deceive us and the aforementioned deceptions will be undetected and remain undetected. Therefore, knowledge must be true justifiable belief and we can never justify our claims to knowledge. The very possibilities of the former two points are sufficient proof that we cannot be sure of what we experience

Heraclitus - everything is in flux - "you cannot step into the same river twice"

Xenophanes questioned the existence of any criterion of true knowledge - if you came across the truth, you would be unable to distinguish it from error

Protagoras - "man is the measure of all things" - i.e there is no true knowledge, each man's views are equally valid

Sextus Empericus - we could never know the real nature of things because:

• Animals perceive things differently
• Different people perceive things differently
• Man's senses perceive the same object in different ways
• Different circumstances cause us to perceive the same object in different ways

Animals perceive things differently

- bats see through sonar

- dogs see shades of red and blue

-fish under the sea see more colours/shades

-snakes see in heat vision/with vibrations

Different people perceive things differently

- different people can perceive a colour as something else. A pink object may seem purple or magenta to different people. Moreover, many people are colour-blind, therefore, colours are different.

-in The Colour of Magic by Terry Pratchett, Wizards can see an eighth colour; the colour of octarine. Perhaps different people, biologically, can see different things.

 taste buds are different; one thing may seem tasty to one person, and disgusting to the next. Moreover, one thing may seem sweet to one person and not sweet to another

Man’s senses perceive the same objects in different ways

- art; many people may think it’s good, others may think it’s terrible. The same for music

Different circumstances cause us to perceive the same object in different ways

- intoxication

- art, you may see it once and it seems good. The next day, it may seem horrid

- if you have a cup of tea in the morning, it might be better than how your tea is later in the day

- optical illusions

Descartes responded to sceptical arguments by claiming that things that are "clear and distinct" are true, and we can rely on them because "God is no deceiver of man". 

Sceptics argued with Descartes, pointng out that we cannot know if something is clear and distinct. We can only tell what appears to be clear and distinct.

Moreover, Descartes is being contradictory. As, he claims that "cogito ergo sum" - all he knows that is that he exists - he cannot trust anything else.

thought that sceptisicm conflicted with common sense. Common sense should be regarded as knowledege until proved not to be. 

"here is one hand, and here is another" - sufficient to prove the existence of a material world

He did not accept the possiblity of illusion 

Although some facts may be beyond our grasp, we do observe the world directly and objects to generally eist as we perceive them