1 00:00:07,179 --> 00:00:11,149 Imagine a brilliant neuroscientist named Mary. 2 00:00:11,149 --> 00:00:13,830 Mary lives in a black and white room, 3 00:00:13,830 --> 00:00:16,220 she only reads black and white books, 4 00:00:16,220 --> 00:00:20,580 and her screens only display black and white. 5 00:00:20,580 --> 00:00:26,160 But even though she has never seen color, Mary is an expert in color vision 6 00:00:26,160 --> 00:00:31,221 and knows everything ever discovered about its physics and biology. 7 00:00:31,221 --> 00:00:33,421 She knows how different wavelengths of light 8 00:00:33,421 --> 00:00:36,842 stimulate three types of cone cells in the retina, 9 00:00:36,842 --> 00:00:38,601 and she knows how electrical signals 10 00:00:38,601 --> 00:00:42,821 travel down the optic nerve into the brain. 11 00:00:42,821 --> 00:00:45,232 There, they create patterns of neural activity 12 00:00:45,232 --> 00:00:50,851 that correspond to the millions of colors most humans can distinguish. 13 00:00:50,851 --> 00:00:52,192 Now image that one day, 14 00:00:52,192 --> 00:00:54,802 Mary's black and white screen malfunctions 15 00:00:54,802 --> 00:00:57,580 and an apple appears in color. 16 00:00:57,580 --> 00:00:58,691 For the first time, 17 00:00:58,691 --> 00:01:03,502 she can experience something that she's known about for years. 18 00:01:03,502 --> 00:01:05,211 Does she learn anything new? 19 00:01:05,211 --> 00:01:10,342 Is there anything about perceiving color that wasn't captured in all her knowledge? 20 00:01:10,342 --> 00:01:13,491 Philosopher Frank Jackson proposed this thought experiment, 21 00:01:13,491 --> 00:01:17,069 called Mary's room, in 1982. 22 00:01:17,069 --> 00:01:21,312 He argued that if Mary already knew all the physical facts about color vision, 23 00:01:21,312 --> 00:01:24,722 and experiencing color still teaches her something new, 24 00:01:24,722 --> 00:01:27,462 then mental states, like color perception, 25 00:01:27,462 --> 00:01:31,713 can't be completely described by physical facts. 26 00:01:31,713 --> 00:01:33,492 The Mary's room thought experiment 27 00:01:33,492 --> 00:01:37,492 describes what philosophers call the knowledge argument, 28 00:01:37,492 --> 00:01:40,041 that there are non-physical properties and knowledge 29 00:01:40,041 --> 00:01:44,852 which can only be discovered through conscious experience. 30 00:01:44,852 --> 00:01:48,024 The knowledge argument contradicts the theory of physicalism, 31 00:01:48,024 --> 00:01:50,603 which says that everything, including mental states, 32 00:01:50,603 --> 00:01:53,684 has a physical explanation. 33 00:01:53,684 --> 00:01:55,813 To most people hearing Mary's story, 34 00:01:55,813 --> 00:01:59,483 it seems intuitively obvious that actually seeing color 35 00:01:59,483 --> 00:02:03,063 will be totally different than learning about it. 36 00:02:03,063 --> 00:02:06,057 Therefore, there must be some quality of color vision 37 00:02:06,057 --> 00:02:09,303 that transcends its physical description. 38 00:02:09,303 --> 00:02:12,822 The knowledge argument isn't just about color vision. 39 00:02:12,822 --> 00:02:18,395 Mary's room uses color vision to represent conscious experience. 40 00:02:18,395 --> 00:02:21,713 If physical science can't entirely explain color vision, 41 00:02:21,713 --> 00:02:26,844 then maybe it can't entirely explain other conscious experiences either. 42 00:02:26,844 --> 00:02:29,304 For instance, we could know every physical detail 43 00:02:29,304 --> 00:02:32,724 about the structure and function of someone else's brain, 44 00:02:32,724 --> 00:02:37,794 but still not understand what it feels like to be that person. 45 00:02:37,794 --> 00:02:42,407 These ineffable experiences have properties called qualia, 46 00:02:42,407 --> 00:02:47,665 subjective qualities that you can't accurately describe or measure. 47 00:02:47,665 --> 00:02:50,469 Qualia are unique to the person experiencing them, 48 00:02:50,469 --> 00:02:51,714 like having an itch, 49 00:02:51,714 --> 00:02:52,934 being in love, 50 00:02:52,934 --> 00:02:54,734 or feeling bored. 51 00:02:54,734 --> 00:02:58,737 Physical facts can't completely explain mental states like this. 52 00:02:58,737 --> 00:03:02,185 Philosophers interested in artificial intelligence 53 00:03:02,185 --> 00:03:03,985 have used the knowledge argument 54 00:03:03,985 --> 00:03:06,715 to theorize that recreating a physical state 55 00:03:06,715 --> 00:03:11,375 won't necessarily recreate a corresponding mental state. 56 00:03:11,375 --> 00:03:12,656 In other words, 57 00:03:12,656 --> 00:03:16,304 building a computer which mimicked the function of every single neuron 58 00:03:16,304 --> 00:03:17,716 of the human brain 59 00:03:17,716 --> 00:03:22,665 won't necessarily create a conscious computerized brain. 60 00:03:22,665 --> 00:03:26,927 Not all philosophers agree that the Mary's room experiment is useful. 61 00:03:26,927 --> 00:03:29,836 Some argue that her extensive knowledge of color vision 62 00:03:29,836 --> 00:03:32,636 would have allowed her to create the same mental state 63 00:03:32,636 --> 00:03:35,446 produced by actually seeing the color. 64 00:03:35,446 --> 00:03:39,655 The screen malfunction wouldn't show her anything new. 65 00:03:39,655 --> 00:03:42,945 Others say that her knowledge was never complete in the first place 66 00:03:42,945 --> 00:03:45,816 because it was based only on those physical facts 67 00:03:45,816 --> 00:03:48,506 that can be conveyed in words. 68 00:03:48,506 --> 00:03:50,085 Years after he proposed it, 69 00:03:50,085 --> 00:03:53,826 Jackson actually reversed his own stance on his thought experiment. 70 00:03:53,826 --> 00:03:56,886 He decided that even Mary's experience of seeing red 71 00:03:56,886 --> 00:04:01,726 still does correspond to a measurable physical event in the brain, 72 00:04:01,726 --> 00:04:05,557 not unknowable qualia beyond physical explanation. 73 00:04:05,557 --> 00:04:07,637 But there still isn't a definitive answer 74 00:04:07,637 --> 00:04:11,037 to the question of whether Mary would learn anything new 75 00:04:11,037 --> 00:04:12,866 when she sees the apple. 76 00:04:12,866 --> 00:04:15,976 Could it be that there are fundamental limits to what we can know 77 00:04:15,976 --> 00:04:18,917 about something we can't experience? 78 00:04:18,917 --> 00:04:21,948 And would this mean there are certain aspects of the universe 79 00:04:21,948 --> 00:04:25,338 that lie permanently beyond our comprehension? 80 00:04:25,338 --> 00:04:30,676 Or will science and philosophy allow us to overcome our mind's limitations?