Unavailable for 30 years, this volume presents the original German/English edition of Wittgenstein's Philosophical Investigations -- one of the most influential philosophical works of the twentieth-century. The last product of intensive work by Wittgenstein during the period 1929-49, Philosophical Investigations explores the concept of meaning, of understanding; of propositions; of logic; of states of consciousness; and of many other topics. The fundamental ideas of the Tractatus are both expounded and criticized.