Formal methods were frequently shown to be effective and, perhaps because of that, practitioners are interested in using them more often. Still, these methods are far less applied than expected, particularly, in critical domains where they are strongly recommended and where they have the greatest potential. Our hypothesis is that formal methods still seem not to be applicable enough or ready for their intended use. In critical software engineering, what do we mean when we speak of a formal method? And what does it mean for such a method to be applicable both from a scientific and practical viewpoint? Based on what the literature tells about the first question, with this manifesto, we lay out a set of principles that when followed by a formal method give rise to its mature applicability in a given scope. Rather than exercising criticism of past developments, this manifesto strives to foster an increased use of formal methods to the maximum benefit.