Science[nb 1] is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe.[nb 2] In an older and closely related meaning, "science" also refers to this body of knowledge itself, of the type that can be rationally explained and reliably applied. Ever since classical antiquity, science as a type of knowledge has been closely linked to philosophy. In the West during the early modern period the term "Science" and "Natural Philosophy" were sometimes used interchangeably to refer to the study of natural phenomena:p.3 and, until the 19th century, natural philosophy (which is today called "natural science") was considered a branch of philosophy. In modern usage "science" most often refers to a way of pursuing knowledge, not only the knowledge itself. 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, including physics, chemistry, geology and biology. It is in the 19th century also that the term scientist began to be applied to those who sought knowledge and understanding of nature..