Gerolamo Cardano

Today he is mainly known as a mathematician. He made some discoveries in algebra. He proposed ways to solve cubic and quartic equations. Niccolò Fontana Tartaglia told him about the solution of one particular cubic equation, $x^3+ax+b=0$. This caused a long fight. His student Lodovico Ferrari solved the quartic equations. Cardano also speaks about imaginary numbers in the same book, Ars Magna. He did not completely understand what they were like though.