Charles Sanders Peirce (1839–1914) proposed a different theory. Unlike Saussure who approached the conceptual question from a study of linguistics and phonology, Peirce, the so-called father of the Pragmatist school of philosophy, extended the concept of sign to embrace many other forms. He considered "word" to be only one particular kind of sign, and characterized sign as any mediational means to understanding. He covered not only artificial, linguistic, and symbolic signs, but also all semblances (such as kindred sensible qualities), and all indicators (such as mechanical reactions). He counted as symbols all terms, propositions, and arguments whose interpretation is based upon convention or habit, even apart from their expression in particular languages. He held that "all this universe is perfused with signs, if it is not composed exclusively of signs". The setting of Peirce's study of signs is philosophical logic, which he defined as formal semiotic, and characterized as a normative field following esthetics and ethics, as more basic than metaphysics, and as the art of devising methods of research. He argued that, since all thought takes time, all thought is in signs, that all thought has the form of inference (even when not conscious and deliberate), and that, as inference, "logic is rooted in the social principle", since inference depends on a standpoint that, in a sense, is unlimited. The result is a theory not of language in particular, but rather of the production of meaning, and it rejects the idea of a static relationship between a sign and what it represents: its "object". Peirce believed that signs are meaningful through recursive relationships that arise in sets of three.