The study deals with interpersonal features in spoken and written scientific texts, and looks for explanations for their use in the social practice of the scientific community. Theoretically, the study shares a common ground with functional linguistics and critical discourse analysis. I will try to find answers for the following questions: What is the contribution of functional and social perspective to the study of language? How do Finnish ways of expressing the degree of certainty and referring to person appear from this point of view? In what way can these features of scientific texts be interpreted as part of the practice of the scientific community? What are the interactional rules of scientific discourse like, and how are they realized in texts? The empirical data of the study consists of seven conference papers on linguistics and of the articles based on these papers. To date, there have been relatively few studies of Finnish scientific discourse, and thus one purpose of my study is to add to our knowledge about the use of Finnish in scientific contexts. The main purpose, however, is to develop a functional model for describing the linguistic features under study, and integrate background knowledge concerning scientific practice and communicative rules of a discourse community with a linguistic analysis of authentic discourse. In this study, the adoption of a functional approach to language has resulted in a model for describing the modification of the degree of certainty and references to persons that differs from traditional linguistic descriptions. Description is based on the function and use of an expression, not on its form, and thus it provides us with a more unified picture of how language expresses these meanings. The analysis also shows that modifying the degree of certainty and referring to person, treated as separate phenomena in the traditional grammars, functionally belong to the same phenomenon: increasing vagueness and evading responsibility. In scientific communicative situations, language offers its user a meaning potential, and means for making strategic choices. In accordance with scientific ideals, a researcher should, for instance, adopt a critical attitude towards previous research, appear as an expert, and present his or her own findings as persuasively as possible. On the other hand, the social rules of interaction in the scientific community require him or her to be tactical and face-conscious in expressing these things. The strategies of hedging and referring to persons belong to a scientist's selection of tactical means.