Question about modern art?

From what I've learned in my modern art history courses, "modern" art dates from the late 1700's to present, and "contemporary" art dates from about 1940 to present. What I don't understand is why the classification for modern art started late 18th century, instead of at the beginning of the Renaissance. That would be a more logical place to separate modern from ancient art (I would think). Does anyone know why "modern" art begins when it does?

Question about modern art?
Labelling art as "modern" art is strongly linked to the Enlightenment period. Before this time (during the Renaissance and periods before) the vast majority of artwork was dedicated to religion. As social and cultural mindset shifted away from theism and towards science artwork began to progress similarly. You can see this shift beginning when you look at Rembrandt's work. He not only paints pieces like "The raising of Lazarus", but he also depicts scenes such as "The Anatomy Lecture of Dr. Nicolaes Tulp". This is what is considered "modern" mentality.
Reply:these are just labels. don't think of Modern as a time period, as in the general world. they just name them as they go. And, we would still have to seperate the art from the 1600's from the "modern" art from the 1800's.