Reasoning is a necessary condition of philosophy. And inertia is shown to be characterized by continuity, which is a necessary condition for reasoning in reality. There is continuity between truth and the corresponding objective things, which has always been an absence in philosophy. It is precisely because of this absence that ambiguity occurs when a reasoning process in reality reaches between being and non-being. Thereby leading to the definition for "philosophy" has been in an inconclusive dilemma. Philosophy is the reasons summed up by human beings in the processes of exploring the laws of nature and gradually cognizing truths. These reasons can involve all knowledge, according to whether they exist in reality to distinguish different domains, which can be divided into three parts: science, metaphysics and mathematics. Truth must have absoluteness and immutability, does not exist in reality, and belongs to the category of metaphysics. Therefore, in the category of metaphysics, only those contents that have continuity with the corresponding objective things in reality, have the necessary condition for belonging to philosophy. Once in this way to define what are contained in metaphysics, the definition for "philosophy" will come naturally. In the reality, everything contains two sides that are both opposite and unified. And as "background", metaphysics is also indispensable. Otherwise, it is impossible to clearly distinguish the two opposing sides and reach a consensus, thereby ignoring the continuity that exists between truth and the corresponding objective things. Therefore, the unity of opposites should be a ternary theory. The three are indispensable, cause and effect to each other, co-birth and co-annihilation.