Museums have evolved over time from collections of artifacts and ideas in ancient times to institutions that preserve and educate about human and natural heritage. They grew out of temples and churches in antiquity and the middle ages. The modern museum emerged in the 15th century and became truly public institutions after the French Revolution, with structures now designed to complement their collections. Museums can be purpose built or conversions and come in many types defined by their subject matter such as art, history, science and more. They serve the non-profit purpose of educating society through acquiring, preserving and exhibiting tangible and intangible heritage.