"The term division of labor has, from long usage, become associated in the public mind with manual processes. But productive labor is, in general, both manual and mental and just as there may be division of manual labor so there may be division of mental labor or division of thought. Modern productive methods tend constantly to separate mental labor from manual labor and then to subdivide each into smaller and smaller parts. The subdivision of manual labor is greatly furthered, as has been seen, by the extended use of tools. Subdivision of mental labor on the other hand is hastened by an increase in the amount of knowledge and mental development necessary to successfully perform the work in hand. Thus the mental labor of designing machinery is performed largely apart from the actual production; and this mental labor has become very closely specialized as the scientific basis of engineering has grown. This process of subdivision is greatly hastened in both manual and mental operations by increased quantity since this, of itself, enables the manager to avail himself of the inherent advantages of division of labor already discussed."