It is well-known that Badiou proposes a new connection between mathematics and ontology. In the first place, this is a move internal to the field of philosophy, as his work does not aim to actively contribute theorems and proofs to mathematics proper. Rather, the aim is to show how certain classical ontological/philosophical questions can be approached by examining abstract (but foundational) mathematical theories. This examination remains distinctly philosophical; insights are lifted from the strictly mathematical language and interpreted ontologically. How is this move justified? Perhaps most importantly: How is the relation between mathematics and philosophy understood here? In this short text, I aim to explain how this new assembly of mathematics and ontology is motivated, how it works and why it contributes something to the philosophical field.