Steven Pinker -- How the Mind Works - Hampshire High Italian ...

The Mind's Eye 243left drawing, paint stripe in the right one. All these differences comefrom one box digging where the other one zags!To see so much world in so little image, you have to undo three lawsthat make images from the world. Each needs a mental "expert" to do theundoing. Like stereo vision, these experts work to give us an accurategrasp of the world's surfaces, but they run on different kinds of information,solve different kinds' of problems, and make different kinds ofassumptions about the world.The first problem is perspective: a 3-D object gets projected into a 2-Dshape on the retina. Unfortunately, any projection could have come froman infinite number of objects, so there is no way to recover a shape fromits projection alone (as Ames reminded his viewers), "So," evolutionseems to have said, "no one's perfect." Our shape analyzer plays the oddsand makes us see the most probable state of the world, given the retinalimage.How can a visual system calculate the most probable state of theworld from the evidence on the retina? Probability theory offers a simpleanswer: Bayes' theorem, the most straightforward way of assigning aprobability to a hypothesis based on some evidence. Bayes' theorem saysthat the odds favoring one hypothesis over another can be calculatedfrom just two numbers for each hypothesis. One is the prior probability:how confident are you in the hypothesis before you even look at the evidence?The other is the likelihood: if the hypothesis were true, what isthe probability that the evidence as you are seeing it now would haveappeared? Multiply the prior probability of Hypothesis 1 by the likelihoodof the evidence under Hypothesis 1. Multiply the prior probabilityof Hypothesis 2 by the likelihood of the evidence under Hypothesis 2.Take the ratio of the two numbers. You now have the odds in favor of thefirst hypothesis.1 low does our 3-D line analyzer use Bayes' theorem? It puts its moneyon the object that has the greatest likelihood of producing those lines if itwere really in the scene, and that has a good chance of being in scenes ingeneral. It assumes, as Einstein once said about God, that the world issubtle but not malicious.So the shape analyzer must be equipped with some probabilities