 These two statements concerning the status of different types of knowledge and the requirement to mix “quantities in the abstract” with “some quality of the subject” in physics can aid in making sense of Hobbes’ actual explanatory practice. What does it mean for physics to be “mixed mathematics”? For Hobbes it means that for many explanations one will first establish that some fact is the case by appealing to sense experience, but to give the reason why one must borrow a principle from geometry.