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Finally, a major issue in Problems of Philosophy lies in the fact that, to Russell, all knowledge is built on knowledge by acquaintance, or the things we know through direct, personal experience. Russell accepts a fundamentally Cartesian point of view, which means he accepts that the proper foundation for philosophical inquiry is individual consciousness and perspective. But how can a theory of knowledge be built on private experiences if this theory is supposed to apply to all beings? This problem (among others) bothered Russell, and in his next major epistemological work, Our Knowledge of the External World, he begins to push his inquiry into the public sphere.