We live in a world of fields – mathematical fields, scalar or tensor fields, electric fields or the Earth’s magnetic field – that keeps us bolted to the ground in complete indifference. In this lecture, I would like to show that the very existence of things is also a question of fields that I would like to qualify as ontological. These fields which induce appearances are responsible for the mutability of things and for the topology of multiple and heterogeneous sites that we develop today, such as the multiverse and pluriverse. I will illustrate my points with several examples, and show that to grasp these fields and the real in both its actual and virtual components, diagrammatic thought offers a suitable solution – as the diagram has a machinic and rhizomatic generosity that structure does not have. Finally, I will look at how the concept of field is a “categorification” of space – a concept I borrow from category theory in mathematics – and clarify what “to categorify” means in terms of methodology.