Rena Descartes was a mathematician who lived in France in the 17th century.While lying in bed one cold day he looked up and saw a fly crawling along the ceiling as shown in He wanted to work out a way of describing where the fly was. Rena Descartes was a mathematician who lived in France in the 17th century.While lying in bed one cold day he looked up and saw a fly crawling along the ceiling as shown in He wanted to work out a way of describing where the fly was.