Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Contemporary science is typically subdivided into the natural sciences, which study the material universe; the social sciences, which study people and societies; and the formal sciences, such as mathematics. The formal sciences are often excluded as they do not depend on empirical observations. Disciplines which use science like engineering and medicine may also be considered to be applied sciences. From classical antiquity through the 19th century, science as a type of knowledge was more closely linked to philosophy than it is now and, in fact, in the Western world, the term "natural philosophy" encompassed fields of study that are today associated with science, such as astronomy, medicine, and physics. Though during the Middle Ages in the Middle East, foundations for the scientific method were laid by Ibn al-Haytham in his Book of Optics. While the classification of the material world by the ancient Indians and Greeks into air, earth, fire and water was more philosophical, medieval Middle Easterns used practical, experimental observation to classify materials. In the 17th and 18th centuries, scientists increasingly sought to formulate knowledge in terms of laws of nature. Over the course of the 19th century, the word "science" became increasingly associated with the scientific method itself, as a disciplined way to study the natural world. It was in the 19th century that scientific disciplines such as biology, chemistry, and physics reached their modern shapes. The same time period also included the origin of the terms "scientist" and "scientific community," the founding of scientific institutions, and increasing significance of the interactions with society and other aspects of culture.