Scientific method in biology has been of great interest since the time of the Greek philosopher Aristotle in the fourth century Before the Common Era. Generally, the aim has been to be like the physical sciences in being causal and increasingly showing biology as a topic governed by universal natural laws. However, there has always been a debate about the end directed nature of organisms, which seem to require explanations in terms of final causes. Although these were expelled from the physical sciences in the Scientific Revolution, they remained in biology until Darwin wrote the Origin of Species. Even after that, they remained but today can be seen to be natural and brought on by Darwin's mechanism of natural selection.