Descartes insisted on a unified view of science, and all scientific categories were unified in philosophy. He compared philosophy to a big tree, the root of which is metaphysics, the trunk is physics (natural philosophy), and the branches are applied disciplines such as medicine, mechanics and ethics. He said:

We don't collect fruits from roots and trunks, but from branches and tips. Therefore, the main function of philosophy lies in the independent function of its parts, which we can finally learn.

So, how does philosophy unify all sciences? Descartes' answer is that the unity of science lies not in the research object, but in the method, and philosophy should first study the scientific method. He believes that mathematical methods are universal, but mathematicians have not reflected on mathematical methods, which is the task of philosophers. He said that ancient geometry and contemporary algebra have narrow and obscure limitations. "We should seek another method that includes the benefits of these two sciences without their shortcomings." Descartes called the scientific method "mathematical universalism".

General mathematics applies the most general characteristics of mathematics to other disciplines. There are two general characteristics of mathematics: "quantity" and "order" When these two characteristics are applied to a wider range, they need to be defined philosophically in order to gain more universal significance. Mathematical "measurement" is just a comparison between quantity and quantity. In areas other than mathematics, we can transform metrics into similarities and differences between unmeasurable objects. For example, the objects of philosophical research are not homogeneous, and there is no uniform standard of measurement, but we can grasp the similarities and differences in nature and degree between different objects through the comparison of concepts.

There are two orders in scientific research: one is the synthesis from simple to complex, and the other is the analysis from complex to simple. In mathematics, the research object is homogeneous, and these two methods are reversible. However, in the study of causality in metaphysics, the ultimate cause is infinite God, and the created things are limited. Cause and result are different in nature and series, so the result cannot be directly deduced from the cause. Therefore, metaphysics cannot directly resort to the cause of infinite end, but needs to find a definite and simple starting point, from which knowledge about cause and effect can be constructed. That is to say, the metaphysical method is to analyze and find the definite first principle first, and then deduce the definite conclusion from the first principle by synthesis.