We can see another person wince or laugh, and listen when they describe joy, sadness, fear, or pain. For physicians, understanding and managing pain is one of the hardest parts of caring for patients. In doing so, it joins a broader series of studies asking whether subjective states, including pain, can be measured in quantifiable biological signals. Joy, sadness, fear, fatigue, anxiety, and suffering all depend on subjective experience. We cannot prove that your green looks like my green, or that your pain carries the same emotional impact as mine.