Complex sentence: The principles of integration were formulated independently by Isaac Newton and Gottfried Leibniz in the late 17th century, who thought of the integral as an infinite sum of rectangles of infinitesimal width.
Split into simpler sentences: The principles of integration were formulated independently by Isaac Newton and Gottfried Leibniz in the late 17th century. The founders of calculus thought of the integral as an infinite sum of rectangles of infinitesimal width.