The father took his sons to Italy around 1636 when they were still young. They first travelled to Milan. According to contemporary biographers he served for three years in the Spanish army. During this time he drew marches and battles, fight scenes, landscapes and military costumes. He then abandoned the weapons and studied for some time in Milan with an unidentified sculptor. He moved to Bologna in 1639 where he first entered the studio of Jérôme Colomès, a painter from Lorraine. According to early Italian biographer Filippo Baldinucci Courtois' talent got noticed in Bologna by prominent painters Guido Reni and Francesco Albani. He continued his apprenticeship in Siena, where he studied for some time at the school of Astolfo Petrazzi.