'I know for sure that the sky is blue'

You believe that you know for sure that the sky is blue. Actually the sky is not blue - it's an optical illusion. The sky has no colour really - it just looks that way to us. Click on the back button and try again! (But remember if you could be wrong about a simple thing like that then it's absurd to trust your own reasoning totally, especially when it comes to things that have eternal consequences.)

The question is - How do you know your reasoning is valid?

Alternatively, instead of clicking the back button you may click here in order to be taken back a page.

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