Nietzsche argues for a perspectivism -- the view that
human beings in some fundamental way create the world of their experience
(rather than passively and accurately "receive" a picture of the external world which is true so far as it corresponds to that external world), so that
"truth" depends in large measure on the perspective from which that "truth" is perceived.
In the first four aphorisms of The Gay Science, Nietzsche further provides several pairs of such perspectives, beginning with his "benevolence or ... evil eye," through common and noble, to "moral" (the "normal" perspective of a received moral or religious tradition) and "extra-moral" (the "large-scale accounting," from which perspective what is "evil" in the "moral" perspective is good in terms of preservation of the species).
Such perspectivism includes not only religion and morality; it further includes a phenomenon thematic to the canon of Western literature -- namely, the male creation of the image of woman. It even includes the last bastion of 19th ct. realism -- namely, the natural sciences (including Newtonian physics) as putatively offering us an accurate, objective picture (and control) of external reality. Even here, on Nietzsche's view, "We operate only with things that do not exist: lines, planes, bodies, atoms, divisible time spans, divisible spaces," -- which are our creation, not "external reality." (172)
To realize perspectivism -- after millennia of believing that we "know" an external, independent reality (either in the form of a God who reveals fundamental rules of right and wrong, or in the form of a clockwork universe, whose rules we attempt to discern via the natural sciences -- leads, in Nietzsche's view, to the greatest of crises. Not only is God "dead" (and we have killed him, precisely as we realize that we are at least partial sources of these "external realities" we have come to depend on): most fundamentally, the familiar and prevailing notions of meaning, truth, and purpose to existence assume an "anchor" in such an objective, external reality. But if we cannot know any such external reality -- what meaning, truth, and purpose to existence is possible? So it is that God is dead, and we've unchained the earth from its sun. There is no longer any up or down -- i.e., no longer any frame of reference by which we can anchor and orient our lives' meaning and purpose.
In this way, Nietzsche culminates the sense of crisis we can see building in modernity, beginning with
the first Romantic critique of Enlightenment rationalism (which was to found a new, democratic utopia, based on humanity's own, rational assessment of human nature, ethics, politics, and power over nature via technology), followed by
Realism's critique of Romanticism (whose emphasis on feeling leads both in democratic [Douglass, Daly] and authoritarian directions) --
while [literary] Realism offers no clear ethical or political guidelines (since, from a "scientific" view, the world is material and deterministic, and thus empty of more traditional sources of meaning -- e.g., human freedom, God, an afterlife, etc.).
If science likewise "falls" in the face of a Nietzschean critique -- what is left to turn to for meaning and purpose?
II. This sense of crisis, moreover, is at least partially reinforced in the late 19th ct. and 20th ct. by developments in the sciences themselves. Beginning with Einstein's relativity theories, and then the developments of Quantum Mechanics, some of the Nietzschean lessons are reinforced -- i.e.,
scientific truth (what is measurable via sense and instrument, quantifiable, expressible in mathematical terms, etc.) depends on the frame of reference (or perspective) of the observer (even with regard to what had been assumed to be universal and absolute, namely time), and
human knowledge of "Reality as it is in itself," after a point, is impossible to achieve -- insofar as the knowing subject forces the universe to "appear" in certain ways, depending precisely on how s/he chooses to measure that universe. Indeed, in fundamental ways, it appears that our human constructs (e.g., "wave" and "particle" as contradictory concepts) simply cannot apply to subatomic phenomena. And, at bottom, the universe seems built in ways that include randomness, probability, and systems that in principle are unpredictable -- contrary to the neat and, in principle, fully predictable Newtonian "clockwork" universe.
Hence, the control and predictability promised by science (via technology), along with claims that science alone can provide us with a certain and "objective" knowledge, as major themes of modernity (from the Enlightenment through 19th ct. Realism and positivism), is now seen to be limited -- not by a lack of information or insufficient instruments, but rather by the way the world is built.
Especially since the natural sciences seemed a last bulwark over against the apparent failures of Enlightenment rationalism, Romanticism, etc. -- the realization that the sciences, too, cannot live up to all their promises, contributes to a sense of cultural crisis in the 20th century.
This sense is captured powerfully in W.B. Yeats' poem "The Second Coming" -- and reinforced by a century which has witnessed two World Wars, the Holocaust, atomic bombings of civilian population centers, the collapse of Soviet communism (as the "other" great Enlightenment experiment in democracy, economic organization, and the march to utopia via technology), and the threat of environmental disaster through rain-forest destruction, rapid extermination of species, etc.