according to rocca, cullen was known for systemizing and promoting medical knowledge rather than producing original research. despite the lack of original work, some believed that cullen's attempt to organize existing knowledge was actually a sign of his "practical sagacity" as a practitioner. he was a lecturer for more than forty years. in order to understand cullen's medical teaching at the time, it is important to understand cullen's conception of "system" as he taught it in his classes. cullen described a system as "an organised body of opinions on particular topics in the medical curriculum." he also referred to system as the principles in his book "first lines of the practice of physic." there were many possible reasons behind cullen's emphasis of the system of medicine. in the 18th century, a period of scottish enlightenment, there were competing theories about the mechanisms of the human body and the causes of diseases proposed by different professors, who competed for students' teaching fees. thus, having an underlying system of medical knowledge was a practical way to organize the knowledge coherently for the students.