I recognize the lion by his claw.
[After reading an anonymous solution to a problem that he realized was Newton's solution.]
But just as much as it is easy to find the differential of a given quantity, so it is difficult to find the integral of a given differential. Moreover, sometimes we cannot say with certainty whether the integral of a given quantity can be found or not.
A quantity which is increased or decreased by an infinitely small quantity is neither increased nor decreased.