Of the military texts written before the unification of China and Shi Huangdi's subsequent book burning in the second century BC, six major works have survived. During the much later Song dynasty, these six works were combined with a Tang text into a collection called the "Seven Military Classics". As a central part of that compilation, "The Art of War" formed the foundations of orthodox military theory in early modern China. Illustrating this point, the book was required reading to pass the tests for imperial appointment to military positions.