Philosophy as a self-perpetuating lie
Many centuries ago, philosophy used to contain all areas of knowledge, including mathematics, natural sciences, psychology and so on. It is curious that, as soon as some branch of knowledge has developed enough maturity, it separated itself from philosophy. The "maturity" means that a branch of knowledge finds a set of criteria for truth and values, and a set of methods that seem to be productive for obtaining actually useful results. Once mathematicians found a way of formulating clear results and computations, they ceased to be philosophers. Once physicists found a method of performing precise experiments and formulating theories, they ceased to be philosophers. The same thing happened to astronomers, chemists, psychologists, and others. The latest discipline to be ejected from philosophy is formal logic. Philosophers who are unable or unwilling to use the good and useful methods developed by a given discipline, nevertheless, continue to philosophize about it.

So, it seems that philosophy is the discipline that, time and again, has ejected from itself any sub-discipline that discovers a coherent and practically useful criterion of truth. What remains is a community of people who continue to engage in fruitless discussions about the "principles" of metaphysics, ethics, and politics, always with a pretense of depth and all-encompassing generality. But nothing ever comes out of these discussions. Philosophy is an eternal, self-perpetuating lie.