To better understand and comprehend the truths of a science, men have created textbooks ("Lehrbuch"), which of course contain only the true propositions of the science known to men. But how to know where to divide our knowledge, that is, which truths belong together? Bolzano explains that we will ultimately know this through some reflection, but that the resulting rules of how to divide our knowledge into sciences will be a science in itself. This science, that tells us which truths belong together and should be explained in a textbook, is the "Theory of Science" ("Wissenschaftslehre").