Virchow believed that diseases arose not in the organs or tissues in general, but in their individual cells. Rejecting the belief, dating back to the Greeks, that diseases were caused by an imbalance of the four humors of the body (blood, phlegm, yellow bile, and black bile), Virchow became a leading spokesman for the new generation of German physicians in the mid-19th century. Virchow envisioned medical progress from three main sources: clinical observations, animal experimentation, and pathological anatomy, especially at the microscopic level.