The great developments in the sciences since the nineteenth century have created the need to find a new role for philosophy. Several attempts have been made in this direction, but they have failed. This raises the question: Can philosophy be useful again, and what type of philosophy can be such? The main thesis of the book is that philosophy can be useful again if and only if it is acquisition of knowledge, that is, an inquiry aimed at acquiring knowledge of the world. The task of the book is to formulate the view that philosophy is acquisition of knowledge, to show that the arguments against it are not valid, and to develop the view by considering several questions about knowledge.