Science (from Latin scientia, meaning "knowledge") is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe. [3] In an older and closely related meaning, "science" also refers to a body of knowledge itself, of the type that can be rationally explained and reliably applied. A practitioner of science is known as a scientist. Since classical antiquity, science as a type of knowledge has been closely linked to philosophy. In the early modern period the words "science" and "philosophy" were sometimes used interchangeably.[4] By the 17th century, natural philosophy (which is today called "natural science") was considered a separate branch of philosophy.