The World According to Phigs

There was a time when I pondered long and hard over a term in Beckett's Murphy and it was this:

Murphy's mind pictured itself as a large hollow sphere hermetically sealed to the universe without. This was not an impoverishment for it excluded nothing that it did not itself contain...

The first part was fine, I didn't have a problem with the first part. It was the second part that was the problem. "excluded nothing that it did not itself contain..."  Even by applying the friend of a friend is a friend rule, there was no way I could resolve these double negatives. On occasion I would call on some of the finest minds in the area, but to no avail. No-one, it seems, could unravel this phrase and soon the conversation would turn to other urgent matters of the day. Wise men are often thus.
And today, as the senility surges I am still none the wiser. Older but not wiser. I still don't know what it means.