Imagine a brilliant neuroscientist named Mary. Mary lives in a black and white room, she only reads black and white books, and her screens only display black and white. But even though she has never seen color, Mary is an expert in color vision and knows everything ever discovered about its physics and biology. She knows how different wavelengths of light stimulate three types of cone cells in the retina, and she knows how electrical signals travel down the optic nerve into the brain. There, they create patterns of neural activity that correspond to the millions of colors most humans can distinguish. Now imagine that one day, Mary’s black and white screen malfunctions and an apple appears in color. For the first time, she can experience something that she’s known about for years. Does she learn anything new? Is there anything about perceiving color that wasn’t captured in all her knowledge? Philosopher Frank Jackson proposed this thought experiment, called Mary’s room, in 1982. He argued that if Mary already knew all the physical facts about color vision, and experiencing color still teaches her something new, then mental states, like color perception, can’t be completely described by physical facts. The Mary’s room thought experiment describes what philosophers call the knowledge argument, that there are non-physical properties and knowledge which can only be discovered through conscious experience. The knowledge argument contradicts the theory of physicalism, which says that everything, including mental states, has a physical explanation. To most people hearing Mary’s story, it seems intuitively obvious that actually seeing color will be totally different than learning about it. Therefore, there must be some quality of color vision that transcends its physical description. The knowledge argument isn’t just about color vision. Mary’s room uses color vision to represent conscious experience. If physical science can’t entirely explain color vision, then maybe it can’t entirely explain other conscious experiences either. For instance, we could know every physical detail about the structure and function of someone else’s brain, but still not understand what it feels like to be that person. These ineffable experiences have properties called qualia, subjective qualities that you can’t accurately describe or measure. Qualia are unique to the person experiencing them, like having an itch, being in love, or feeling bored. Physical facts can’t completely explain mental states like this. Philosophers interested in artificial intelligence have used the knowledge argument to theorize that recreating a physical state won’t necessarily recreate a corresponding mental state. In other words, building a computer which mimicked the function of every single neuron of the human brain won’t necessarily create a conscious computerized brain. Not all philosophers agree that the Mary’s room experiment is useful. Some argue that her extensive knowledge of color vision would have allowed her to create the same mental state produced by actually seeing the color. The screen malfunction wouldn’t show her anything new. Others say that her knowledge was never complete in the first place because it was based only on those physical facts that can be conveyed in words. Years after he proposed it, Jackson actually reversed his own stance on his thought experiment. He decided that even Mary’s experience of seeing red still does correspond to a measurable physical event in the brain, not unknowable qualia beyond physical explanation. But there still isn’t a definitive answer to the question of whether Mary would learn anything new when she sees the apple. Could it be that there are fundamental limits to what we can know about something we can’t experience? And would this mean there are certain aspects of the universe that lie permanently beyond our comprehension? Or will science and philosophy allow us to overcome our mind’s limitations?