Things refer to the objects and phenomena in reality. Limits refer to the truths and the processes of approaching them. Then there is the principle of inertia, which is characterized by continuity, and a necessary condition for reasoning in reality. Therefore, things, limits and inertia are the three elements that constitute "The Theory on Thing's Limits". The truth must have absoluteness and immutability, does not exist in reality, and belongs to the category of metaphysics. The characteristic of the truths is that they cannot be proved by empirical methods, and can only be gradually approached by repeated practices. Based on this, this article has established the norm for identifying truth. It not only has achieved the unity of opposites of all knowledge, but can also according to have the characteristic of continuity on the change of objective things make bidirectional reasoning under the premise of mutual restrictions. From the quantitative change in the real space has gone deep into the qualitative change of ideal realm, it extends the philosophy of materialism to the category of metaphysics. As a result, the true nature of metaphysics has been restored. That is to say, it has neither divorced from practices, nor just observed objective things with a one-sided, isolated and static way of thought. The theory is applicable to all academic categories, and it is to provide an effective method for testing authoritative theories, clarifying chaos, and deriving new knowledge. Finally, according to the theory and norm therein, combining with the objective status quo, this article briefly has analyzed three examples and pointed out the crux of Einstein's special relativity.