We should use physics to understand the laws existing in the depths of nature. When considering what physics is, this idea cannot be ignored. The further development of physics has not only expanded the scope, but also included the development of mechanics into light, heat, electromagnetism, atoms, molecules and even chemistry. There is a tendency to unify all phenomena and integrate all disciplines, which we might as well call the theory of unity as opposed to the theory of differentiation. The famous physicist Rutherford's famous saying: "All sciences are either physics or chemistry."

Such a simple definition is convenient and concrete for scientists who are actually engaged in scientific research, but it is a bit confusing for the public. This means that when scientists talk about science, when ordinary people talk about science, and even when researchers from different disciplines (such as natural science and humanities) talk about science, their definitions may be different.