The article is devoted to plagiarism as a subject of linguistic expertise; it considers the legislative and mundane concepts of plagiarism, of various variants of the terminological designation of this phenomenon in documents of various stylistic affiliations, emphasizes the absence of a clear definition of plagiarism in the linguistic and legal literature. The absence of a generally accepted definition of plagiarism is the cause of many copyright offenses. The article informs that technical means and special anti-plagiarism programs cannot always correctly identify the facts of plagiarism. In such cases, linguistic expertise is needed to detect unauthorized borrowings. Moreover, Russian legislation only protects text copyrights, leaving idea authorship unprotected. The article proposes a method for identifying plagiarism in scientific texts based on a logical by-sentence analysis. The logical sentence analysis is based on the method proposed by the Russian linguist V. Z. Panfilov. The method helps to identify not only borrowed text fragments, but the ideological similarity of compared texts. The logical analysis method is used in the article to compare two contexts of scientific content, which have different grammatical structure and different lexical composition. It reveals clear propositions underlying the scientific text and makes it possible to determine whether the texts compared have identical content. Although cumbersome in interpretation, the proposed analysis can produce very tangible results and provide facts sufficient for incorrect borrowing identification.