A discussion of a number of facets of the analytical theory of science concerning the explication of the relation of scientific knowledge and experience. The central view is that, in order to be accepted as valuable or at least falsifiable, scientific theories must be indicated by 'observational sentences'. But when can a sentence be considered such? Considerations of language and meaning are central in this book; in the 4th chapter Wittgenstein's theory of meaning is taken as the vantage point.