This book provides a philosophy of mathematics that resonates with critical mathematics education. It draws attention to the social complexities that characterise the period of Modernity including the extreme exploitation of manual workers and their families, brutal forms of colonisations, trading of slaves, and the formation of racist ideologies. It portrays Modernity as a period of contradictions and highlights that mathematics is a part of these contradictions. The text interprets mathematics as being indefinite, seeing that mathematics is a part of the ongoing human constructions. It outlines a performative interpretation of mathematics, portraying mathematics as intrinsically connected to actions. Any kind of action, including mathematics-based actions, calls for ethical considerations and this leads to any mathematical practice, either in research, application, or education, to face ethical challenges. It illustrates with examples how a critical philosophy of mathematics can come to be an integral part of classroom practices.