Page 157 - Foundations Of Differential Calculus

140 7. On the Differentiation of Functions of Two or More Variables
certainty that there is no function of x, y, and z whose differential is Pdx+
Qdy + Rdz. For such a formula we cannot require an integral, and such
is said not to be integrable. It can easily be understood that in integral
calculus we ought to investigate the differential formulas to see whether
they can be integrated before actually working to find the integral.