quite a while back now
As Kant pointed out quite a while back now, no metaphysical axiom can be proven to be necessary.
what does it mean by "quite a while back now"? "a while back " is slangy? Thanks

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Seriously, Kant + quite a while back....
Isn't the meaning obvious?????

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"A while back" is not slangy. However, it is colloquial, which is quite different from slang.

It basically means "some time ago".  "Quite a while back now" could be expressed as "a fair amount of time ago, from today".