In the 19th century, Dutch opthalmologist Franciscus Donders assumed that the total time to complete a mental task was the summed duration of each component mental operation. To isolate a mental operation, Donders calculated the difference between the time required to execute a task and the time required to execute the same task when a hypothesized component operation was appended. Several landmark findings in cognitive psychology owe a debt to the development of this tremendously influential method, including the Stroop effect AND SEVERAL OTHER STUDIES I WILL NOT NAME BECAUSE THEY SHOULD BE ON THIS WIKI.
Donders, F. C. (1868). On the speed of mental processes. Arch. Néerland. 3, 269–317.
Gottsdanker, R. & Shragg, G.P. (1985). Verification of Donders' subtraction method. Journal of Experimental Psychology: Human Perception and Performance, 11(6), 765-776.