Science is a systematic enterprise that builds and organises knowledge in the form of testable explanations and predictions about the universe. 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 words "science" and "philosophy of nature" were sometimes used interchangeably and until the 19th century natural philosophy (which is today called "natural science") was considered a branch of philosophy.Nature, in the broadest sense, is equivalent to the natural, physical, or material world or universe. "Nature" refers to the phenomena of the physical world, and also to life in general. It ranges in scale from the subatomic to the cosmic. The study of nature is a large part of science. Although humans are part of nature, human activity is often understood as a separate category from other natural phenomena.