Holm Tetens attempts to define ‘god’ as part of his rational theology. The term ‘god’ is supposed to be defined as ‘the infinite I-subject’. This should be achieved through the customary definition of a singular term by description. However, definitions of this kind have to meet certain formal requirements, which a Rational Theology should adhere to. This paper aims to show that Tetens’ definition faces problems with these requirements since vital arguments are either missing or incomplete in a substantial way. In the process, possible approaches to remedy the outlined problems are hinted at. Overall, this paper raises the question of an alternative interpretation of his definition of god, which is central to the author’s project.