In Philosophical Investigations, Wittgenstein aims to demotivate philosophical theorizing by examining the conditions under which philosophical puzzlement arises. His goal is to enact this `therapy' without advancing controversial philosophical theories himself. The implementation of this new methodology distinguishes the late Wittgenstein from the early Wittgenstein of the Tractatus. After completing work on the Investigations, Wittgenstein continued to write philosophical remarks, including those published in On Certainty, until his death in 1951. Recently, some interpreters have called for the recognition of a third phase of Wittgenstein's career associated with On Certainty, during which Wittgenstein purportedly lost interest in the therapeutic goals of his second phase and adopted a systematic approach to classical epistemological problems. In this dissertation I challenge the idea of a `third Wittgenstein' by arguing that Wittgenstein retained his therapeutic aims in On Certainty - although he was not always successful in fulfilling his methodological goals. A survey of Wittgenstein's correspondence reveals that he consistently criticized the quality of his writing throughout the year 1950. Yet in the spring of 1951, just weeks before his death, Wittgenstein reported that he had regained his philosophical capacities and was doing his best work in years. These fluctuations in Wittgenstein's assessment of his writing correspond to the dates he underwent cancer treatments that affected his cognitive abilities. The results of philological investigation show that the first half of On Certainty was written during Wittgenstein's self-critical phase, while the second half was written during his final weeks of satisfactory work. The early remarks of the book contain a response to G.E. Moore's attempt to refute skepticism that is based on a theory of `hinge propositions'. Later in the book Wittgenstein implements a more therapeutic, less dogmatic method in his treatment of Moore. By exploring the ways that Moore's philosophical assertions can be used in everyday contexts, Wittgenstein wishes to lead us to question whether we fully understand what Moore is trying to say. I argue that Wittgenstein was satisfied by this latter response to Moore because it fulfilled the therapeutic and anti-theoretical aims of his later philosophy.