In this portrait, Dakota Harr whimsically imagines Karl Pearson as a Hamlet-like figure. Pearson plays a central role in The Book of Why, because he was to a considerable degree responsible for expunging causal language from statistics. He categorically insisted that causality has no scientific meaning, and that statistics should limit itself to the extraction of patterns from data. Pearson was fascinated by craniometry and did a study of the skulls in the Paris Catacombs, so this portrait is not too far-fetched!

Robert Frost’s poem expresses a concept that is central to causal reasoning: counterfactuals, or “might-have-beens.” If I had taken the other path in the woods, how would my life have been different? Maayan Harel beautifully illustrates this age-old question — which, with a sufficiently well-defined causal model, can be given a scientific answer.

To be useful for decision-making, a causal model should be able to predict the result of an intervention. For example, if I raise tariffs on olives, what will be the effect on the price of olive oil? In this “map” drawn by Dakota Harr, we offer four different routes toward answering such questions. Two of them (front-door adjustment and do-calculus) were discovered by my co-author, Judea Pearl, and his students.

Planetary scientist, artist, and writer Bill Hartmann is a pivotal figure in my book The Big Splat, or How Our Moon Came to Be. Hartmann proposed the giant impact theory of the moon’s formation, which is now accepted by most experts. In fact, it is believed that most of the planets experienced giant impacts during their formation, which accounts for the diversity of their inclinations, rotational periods, and of course moons. In part, Hartmann credits his artistic temperament for his discovery, because it makes him look at the big picture that other scientists sometimes miss. Here, he is painting an exoplanet in his studio in Tucson, Arizona.

This picture, without the buggy eyes, appeared in my prize-winning article “A Tisket, A Tasket, An Apollonian Gasket,” which appeared in the January 2010 issue of American Scientist. It shows an Apollonian gasket, which results when three circles are drawn mutually tangent to one another and then the pores between them are filled with new circles ad infinitum. In this gasket, nicknamed “bugeye” by Princeton undergraduate Katherine Sanden (who drew the picture), the first three circles have radius 1, 1/2, and 1/2. The next two have radius 1/3, then the next four have radius 1/6, the next four have radius 1/11, and so on. Is there a pattern?