The Book of Why: The New Science of Cause and Effect

by Judea Pearl

The idea did not come to me in a dream; it came from an article by David Rumelhart, a cognitive scientist at University of California, San Diego, and a pioneer of neural networks. His article about children’s reading, published in 1976, made clear that reading is a complex process in which neurons on many different levels are active at the same time (see Figure 3.4). Some of the neurons are simply recognizing individual features—circles or lines. Above them, another layer of neurons is combining these shapes and forming conjectures about what the letter might be. In Figure 3.4, the network is struggling with a great deal of ambiguity about the second word. At the letter level, it could be “FHP,” but that doesn’t make much sense at the word level. At the word level it could be “FAR” or “CAR” or “FAT.” The neurons pass this information up to the syntactic level, which decides that after the word “THE,” it’s expecting a noun. Finally this information gets passed all the way up to the semantic level, which realizes that the previous sentence mentioned a Volkswagen, so the phrase is likely to be “THE CAR,” referring to that same Volkswagen. The key point is that all the neurons are passing information back and forth, from the top down and from the bottom up and from side to side. It’s a highly parallel system, and one that is quite different from our self-perception of the brain as a monolithic, centrally controlled system.