Like contemporary anti-Cartesians, Nietzsche believes that the idea of objective knowledge as a “view from nowhere” is incoherent. He tries to bring this out by comparing knowledge as we actually have it to seeing a visual image, which is “perspectival” in a more straightforward but analogous way. When I look at something, I have to stand somewhere, and so what I see is only the image as it appears from that angle. I cannot see the back (or maybe even the front), nor can I see the whole thing at once.
In this sense, no perspective is “truer” than any other. If a snapshot taken from that angle results in an inaccurate image, this must be due to a faulty camera, not my standpoint. Nor can there be such a thing as a complete visual image, say constructed out of all the individual ones. (Of course I may move from one place to another until I have seen enough; but that is not at all the same thing.) Yet I may find one angle more revealing than another, and some of them may be misleading or useless. Still, these judgments and manipulations are after the (photographic) fact: they are not reducible to purely disinterested registering of how things are, visually speaking.
If cognition is like vision in this way, then just as there is no such thing as a single complete visual image, to be seen from no particular vantage point (which yet preserves the idea of accurate or faulty representations of what can be seen from each), then there is no such thing as a single complete way things are for us to know (a “world-in-itself”): all there are are interpretive perspectives and what can be seen from them.
One of my SNR friends uses a similar definition of “God” — she suggests it being something like the sum total of all the knowledge in the universe. But, as I tried to argue with her, knowledge in and of itself is senseless; knowledge means nothing without a knower, an interested perspective, a particular point of view. Facts matter to us not for what they are, but what they’re for.