Title: Formulating and Proving the Trend of DNNs Learning Simple Concepts. Abstract: This paper theoretically explains the intuition that simple concepts are more likely to be learned by deep neural networks (DNNs) than complex concepts. Beyond empirical studies, our research first specifies an exact definition of the complexity of the concept that boosts the learning difficulty. Specifically, it is proven that the inference logic of a neural network can be represented as a causal graph. In this way, causal patterns in the causal graph can be used to formulate interactive concepts learned by the neural network. Based on such formulation, we explain the reason why simple interactive concepts in the data are more likely to be learned than complex interactive concepts. More crucially, we discover that our research provides a new perspective to explain previous understandings of the conceptual complexity. The code will be released when the paper is accepted.