The aim of this chapter is to show how logistic analysis has altered the approach to philosophical problems. We will see how Russell turned the epistemological problem of the external world into a question of comparing the different languages of constructional systems. But first we must explain the logicist definition of number, which is the prototype of a further kind of russellian analysis, that of so-called reduction. A knowledge of this is essential for an understanding of the question that has replaced the problems of the theory of knowledge, viz. the question of so-called "unnecessary entities".