Science is a rigorous, systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the world. Modern science is typically divided into three major branches: the natural sciences, which study the physical world; the social sciences, which study individuals and societies; and the formal sciences, which study formal systems, governed by axioms and rules. There is disagreement whether the formal sciences are science disciplines, as they do not rely on empirical evidence. Applied sciences are disciplines that use scientific knowledge for practical purposes, such as in engineering and medicine.