The article discusses the emergence of a scientific theory of deep learning, which aims to characterize properties and statistics of the training process, hidden representations, and performance of neural networks. It identifies five growing bodies of work that contribute to this theory, including solvable idealized settings, tractable limits, and simple mathematical laws. The authors argue that this theory is best thought of as a mechanics of the learning process, which they term 'learning mechanics'. This theory is expected to provide a deeper understanding of deep learning and its applications.