The Dartmouth Summer Research Project on Artificial Intelligence, held from 18 June through 17 August of 1956, is widely considered the event that kicked off AI as a research discipline. Organized by John McCarthy, Marvin Minsky, Claude Shannon, and Nathaniel Rochester, it brought together a few dozen of the leading thinkers in AI, computer science, and information theory to map out future paths for investigation.

A group photo [shown above] captured seven of the main participants. When the photo was reprinted in Eliza Strickland’s October 2021 article “The Turbulent Past and Uncertain Future of Artificial Intelligence” in IEEE Spectrum, the caption identified six people, plus one “unknown.” So who was this unknown person?

Who is in the photo?

Six of the people in the photo are easy to identify. In the back row, from left to right, we see Oliver Selfridge, Nathaniel Rochester, Marvin Minsky, and John McCarthy. Sitting in front on the left is Ray Solomonoff, and on the right, Claude Shannon. All six contributed to AI, computer science, or related fields in the decades following the Dartmouth workshop.

In the back row from left to right are Oliver Selfridge, Nathaniel Rochester, Marvin Minsky, and John McCarthy. In front on the left is Ray Solomonoff; on the right, Claude Shannon. The identity of the person between Solomonoff and Shannon remained a mystery for some time.The Minsky Family

Between Solomonoff and Shannon is the unknown person. Over the years, some people suggested that this was Trenchard More, another AI expert who attended the workshop.

I first ran across the Dartmouth group photo in 2018, when I was gathering material for Ray’s memorial website. Ray and I had met in 1969, and we got married in 1989; he passed away in late 2009. Over the years, I had attended a number of his talks, and I had met many of Ray’s peers and colleagues in AI, so I was curious about the photo.

I thought, “Gee, that guy in the middle doesn’t look like my memory of Trenchard.” So I called up Trenchard’s son Paul More. He assured me that the unknown person was not his father.

More recently, I discovered a letter among Ray’s papers. On 8 November 1956, Nat Rochester sent a short note and a copy of the photo to some colleagues: “Enclosed is a print of the photograph I took of the Artificial Intelligence group.” He sent his note to McCarthy, Minsky, Selfridge, Shannon, Solomonoff—and Peter Milner.

Several months after the workshop, Nathaniel Rochester sent a copy of the photo, along with this note, to six people.Grace Solomonoff

So the unknown person must be Milner! This makes perfect sense. Milner was working on neuropsychology at McGill University, in Montreal, although he had trained as an electrical engineer. He’s not generally lumped in with the other AI pioneers because his research interests diverged from theirs. Even at Dartmouth, he felt he was in over his head, as he wrote in his 1999 autobiography: “I was invited to a meeting of computer scientists and information theorists at Dartmouth College…. Most of the time I had no idea what they were talking about.”

In his fascinating autobiography, Milner writes about his work in radar development during World War II, and his switch after the war from nuclear-reactor design to psychology. His doctoral thesis in 1954, “Effects of Intracranial Stimulation on Rat Behaviour,” examined the effects of electrical stimulation on certain rat neurons, which became widely and enthusiastically known as “pleasure centers.”

This work led to one of Milner’s most famous papers, “The Cell Assembly: Mark II,” in 1957. The paper descri