At bottom, what the investigator of such truths is seeking is only the metamorphosis of the world into man. […] His method is to treat man as the measure of all things, but in doing so he again proceeds from the error of believing that he has these things [which he intends to measure] immediately before him as mere objects.  He forgets that the original perceptual metaphors are metaphors and takes them to be the things themselves.*
(Friedrich Nietzsche)

human understanding at best can understand itself, but nothing outside itself**
(Ursula Le Guin)

No-one is purporting we know all there is to  know, that we have discovered everything there is to discover.  New scientists continue to emerge and are likely to for a long time yet.

We have to accept limitations, though.  We cannot become a bird or a squid or a millipede.  All of which discover and explore and know this world in different ways to us.  We may imagine what their experience is but its human imagining.

This is our weakness and when we accept it, then we shall be stronger.

(*Friedrich Nietzsche, quoted in Maria Popova’s Brain Pickings: Nietzsche on Truth, Lies, the Power and Peril of Metaphors.)
(**From Ursula Le Guin’s Words Are My Matter.)