It is an important basis of ontology to explain clearly what the subjective world contains. Frege and other Western philosophers were seriously wrong about concepts and the foundations of mathematics. The simulation is to mimic something with a fake product. People's knowledge about the world is also a kind of simulation. The content of knowledge is fake product relative to the object. The difference between people's knowledge and the object that is simulated can be made on purpose. The altered knowledge doesn't necessarily have real object corresponding to it. These thought games can bring about more value of knowledge and benefit in practice. The civilization and culture of human being mainly come from the innovative thought games. The logical construct of sentences describing something is a simulation of the thing or thought game. A symbol is a substance used to create an association in mind. Logical constructs have three advantages compared with real things: simple, useful, and stable. The concept is the logical construct produced after abstraction of things. Different degrees of abstraction can produce different degrees of concept. Abstract concepts of most disciplines retain more attributes, so they are not easy to analyze and study. The concepts of numbers and points, lines, planes and bodies are the results that retain few attributes after abstraction, which are simpler than concepts of other disciplines and easier to analyze and study.
Key Words: Epistemology; Logical construct; Concept; Simulation; Maths; Geometry