I am sincerely interested in events or discoveries that can be thought as definitive game changers in the course of philosophy. Russell's Principia Mathematica and its failure, if it can be thought of as philosophy, would be one in my humble opinion.
closed as too broad by virmaior, iphigenie, James Kingsbery, stoicfury Oct 21 '14 at 1:33
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In addition to Russell's and Wittgenstein's analytic philosophy some of the most major influences on modern philosophy, which may perhaps be called breakthroughs, are:
1) Kant's transcendental method and critique of metaphysics that ended speculations about "intelligible" entities, and our supposed ability to perceive them directly. Transcendental arguments from structure of knowledge to faculties of mind became a frequent part of reasoning in philosophy and cognitive sciences.
2) Husserl's phenomenology that built on the idea that there is no qualitative gap between sensory perception of material objects and intuition of ideas. Rather they are idealized extremes of the same ability we possess. This positively complemented Kant's critique to a degree.
3) Existensialism, which goes back to Kirkegaard and Nietzsche, who pointed out that philosophy should draw on life experience as a whole, and that is far broader than rational thought dominating most of the classical philosophy.
4) Hegel's dialectic, a still controversial insight that not just the knowledge itself, but also its relationship to reality, evolve in ways which make traditional static ontologies and epistemologies inadequate. The evolution process is supposed to be universal, and is often likened to a spiral of triads. The original assertion is first "negated", and then "negated" again into a refinement of the original, but at a higher level. No static "final truths" (still retained by Kant, Husserl and existensialists) are available because even the correspondence to reality is itself subject to the dialectic. Hegel's ideas became more famous outside of philosophy due to Marx, who transplanted them into history and politics, where they fed into his theory of social change.
Other influential contributions are structuralism, and Heidegger's hermeneutics, but they are not at the same level as the ones above. Heidegger, for example, creatively combines phenomenology and existensialism to recover some of the lost metaphysics by mitigating the separation between the subject and the object. Structuralism came from developments in linguistics (Saussure) and anthropology (Levi-Strauss) that are similar to earlier mathematization of natural sciences. But the idea that philosophy and science can properly access only relational structures, if that, rather than the essences behind them, goes back to Kant.