Complex sentence: During the early modern period the words "science" and "philosophy of nature" were sometimes used interchangeably and not until the 17th century, natural philosophy (which is today called "natural science") was considered a separate branch of philosophy in the West.
Split into simpler sentences: During the early modern period the words "science" and "philosophy of nature" were sometimes used interchangeably. And not until the 17th century, natural philosophy (which is today called "natural science") was considered a separate branch of philosophy in the West.