Awarded the 2011 Alan Turing Prize in Computer Science, the scientist and statistician shows how understanding causal relationships revolutionized science and made a breakthrough in work on artificial intelligence." Correlation is not causation" - this mantra, chanted by scientists for more than a century, led to a conditional ban on talking about causal relationships. Today this taboo has been abolished. The causal revolution discovered by Judia Pearl and colleagues survived a century of confusion and put causality - the study of cause and effect - on solid scientific footing. Pearl's work allows us not only to know if one is the cause of the other, it allows us to explore the reality that already exists and the realities that could exist. It demonstrates the essence of human thought and provides the key to artificial intelligence.