The impact of cognitive operations on haemodynamic activity in the human brain is a cornerstone of modern cognitive neuroscience. This essay presents an early speculation about why there is increased blood flow following cognitive operations: Emil Harleß, a 19th century German physiologist, proposed that this blood flow responds to irritations caused by "the will" in order to restore homeostasis. Peculiar from a modern perspective, this speculation shows how neuroscientific concepts - and corresponding perspectives on cognitive function - have changed over the centuries.
