Of Grammatology is a nonfiction book by French philosopher Jacques Derrida. The main theme of the work, which was originally a thesis, is that the writing process is seen simply as an extension or medium of speech, which makes writing a derivative of speech. The other theme of the book is the deconstruction of literary works by selected writers, such as Ferdinand de Saussure and Martin Heidegger. However, Derrida merely uses their works in order to clarify and demonstrate his argument, and not to criticize their philosophical positions. For instance, when Derrida deconstructs Saussure's concept of signs—which is part of linguistics structuralism—he quotes Saussure's work, and argues that oral speech and writing are both signifiers. Like Saussure, Derrida believes that writing is a derivative of speech, but argues that written symbols and signs are equivalent to oral speech. This means that from signs, one can deconstruct its semiotics or symbolism and create writings from it in the same way that writing is a derivative of speech. Of Grammatology would influence deconstructive criticism.