The series Too Hard for Science? discusses ideas scientists would love to explore that they think are difficult or impossible to investigate.

The Scientist: Robert Stickgold, director of the Center for Sleep and Cognition at Harvard Medical School.

The Idea: How many memories does a person create in one day? Assumptions regarding this number are at the foundation of many studies of the brain. One could put recording equipment on volunteers and compare what they experienced to what they actually remembered about events, Stickgold suggests. This is something he and his colleagues attempted.

The Problem: One dif­ficulty regards how one counts memories. Is going to the grocery store one memory or a series? Remembrances are fractal in nature: the deeper a person looks into one memory, the more details one can unearth.

Furthermore, Stickgold adds, “There probably really isn’t anything in the brain that’s a discrete memory.” The brain is one vast, interconnected network, so how much you glom together and call a memory—“Well, it’s not a question that’s meaningful at the level of the brain.”