The determinant of species diversity in a community is one of the most fundamental topics in ecology. Although serious concerns have been given, there is very little consensus on this question and other related issues, such as species coexistence, latitudinal diversity gradient, the distribution of species relative abundance and so on. This study treats the community as an energy transfer system, which is supposed to be restricted by the second law of thermodynamics and the rule of increasing entropy. The main purpose is to understand the processes of energy division among species according to the second law and increasing entropy. To this end, a thought experiment is presented and three theoretical conclusions are derived. (1) The theoretical maximum of species diversity is 1, 1/2, 1/3, 1/4… ; (2) Richness is infinite in theory; (3) Species diversity will be affected by the factors that act on the entropy, which is the essence of diversity. These points are tested by empirical samples that include 8,272 quantitative communities from three datasets. If these conclusions can be verified by further investigations, a unified theory of species diversity may be developed toward the nice prospect for the reform of the basic understanding of community.