Institute for Mathematical Behavioral Sciences

Is it possible, John Locke pondered in his Essay of 1690, that “the idea that a violet produced in one man's mind by his eyes were the same that a marigold produced in another man's, and vice versa.”?1 Could the colours I experience differ from yours, even if experiments reveal no difference between us? Locke's question is raised by inquisitive children, but remains hotly debated by scientists and philosophers because its answer is key to an unsolved mystery of science: What is the relationship between brain activity and conscious experience?2–8 How, precisely, can brain activity cause, or be, my experience of chartreuse or the fragrance of a rose?9,10 If colour scrambling of the type Locke envisioned is possible, this would notably constrain the empirical search for relationships between brain activity and consciousness. It would entail that conscious experiences could change without concomitant functional changes in brain states. Locke's question has stirred prolific debate through the ensuing centuries, but no mathematical articulation or proof. Here we prove that the answer is “Yes”: colour scrambling is possible without violating scientific laws.