Post- modern Epistemology - how we know

I was talking a few days ago about post-modern forms of Christianity. They are characterised by a rejection of dogma and what they call unchristian divisiveness. 
 One of the foundational ideas about these days is about relativism.  Thinkers no longer believe in any absolute certainties.  Absolute truth is a myth. It is an intolerable concept.  There is an atmosphere about that is summed up in the ideas of Lyotard (nothing to do with dance or gymnastics) and his rejection of all grand, all-explaining theories (meta-narratives) and all the so-called certainties that were the fruits of the Enlightenment, whether in science or religion or philosophy.  This modernism is out of date and needs to be supplanted by dialogue and tolerance.  The old certainties are gone.
These sceptical ideas go back a long time. There is nothing new under the sun.  The Greek sceptics at the time of Plato are well known.
The two supreme English examples are Berkeley and Hume, both of whom lived at the time of Wesley.  Berkeley was a sincere Christian who published books and dialogues concerning idealism. This doctrine says that we know only what is going on in our own minds. We can be certain of nothing else.  We cannot know whether this bundle of perceptions in our heads refer to any objective reality.  His Latin motto was esse est percipi - existence is based on perception.  If an object is no longer perceived it no longer exists. When challenged by this question, "If an object ceases to be perceived by someone does it cease to exist?",  he replied, "No, it still exists because it is perceived by the mind of God" 


Ronald Knox, a Catholic wit of the last century, wrote an amusing limerick on Berkeley's idealism

There was a young man who said "God

Must find it exceedingly odd

To think that the tree

Should continue to be

When there's no one about in the quad."

"Dear Sir: Your astonishment's odd;

I am always about in the quad.

And that's why the tree

Will continue to be

Since observed by, Yours faithfully, God."
Of course, most of us endowed with a modicum of common sense, will treat all this as a lot of intellectual nonsense.  Dr Johnson, a contemporary of Berkeley, when told of his theory that matter didn't really exist, shouted loudly and said, "I refute it thus!" and kicked a nearby pillar with some force!