"To imagine that a time will come whose composition is, in its specifics, unimagineable to us is to imagine the familiar and the fruit of experience, and so I stick with "irony."
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Nice construction and certainly seemingly ironic.

But then if we use your reasoning the sentence "I can imagine "nothingness" would also be true.

And what then would be this "nothingness"?
The human mind cannot conceptualize the "absence of everything" without a "something"
to contrast it, so what then would we be imagining?

The logic here might be paradoxical but it could not correctly be said to be ironic.
There is a subtle difference.