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.A documentary film is a nonfictional motion picture intended to document some aspect of reality, primarily for the purposes of instruction or maintaining a historical record. Such films were originally shot on film stock—the only medium available—but now includes video and digital productions that can be either direct-to-video, made into a TV show or released for screening in cinemas. "Documentary" has been described as a "filmmaking practice, a cinematic tradition, and mode of audience reception" that is continually evolving and is without clear boundaries.