An external approach to unitary representations

The main aim of this paper is to present the ideas which lead first to the solution of the unitarizability problem for $\GL(n)$ over nonarchimedean local fields and to the recognition that the same result holds over archimedean local fields, a result which was proved by Vogan using an internal approach. Let us say that the approach that we are going to present may be characterized as external. At no point do we go into the internal structure of representations.