Abstract
This paper deals with the problem of the External World, taking its point of departure in Peter Zinkernagel's Conditions for Description. In the first section I try to give an outline of the theses contained in that book. In the second I raise a main objection against it, pointing out that Zinkernagel, in one respect, has not sufficiently sharpened the argumentation between phenomenalism and realism. In the third section I turn realism and phenomenalism sharply against each other, presenting the latter in a radical, yet consistent form; the section is an attempt to show how phenomenalism can be rejected.