Greek philosopher Aristotle (fl. 4th century BCE) found that objects immersed in a medium tend to fall at speeds proportional to their weight. Vitruvius (fl. 1st century BCE) understood that objects fall based on their specific gravity. In the 6th century CE, Byzantine Alexandrian scholar John Philoponus modified the Aristotelian concept of gravity with the theory of impetus. In the 7th century, Indian astronomer Brahmagupta spoke of gravity as an attractive force. In the 14th century, European philosophers Jean Buridan and Albert of Saxony —who were influenced by certain Islamic scholars —developed the theory of impetus and linked it to the acceleration and mass of objects. Albert also developed a law of proportion regarding the relationship between the speed of an object in free fall and the time elapsed.