Capital Vol. 1 Chapter 14: Division of Labour & Manufacture
Work becomes more and more specialised, each worker performing only part of the whole process
This causes "increased intensity of labour" i.e. less labour is lost switching tasks, more labour spent productively
But Marx says something else is lost: the worker is drained of their "vital forces" by the constant repetition of the same task. Workers are denied the human joy of variety & change in their work