(Popular Science) – In 1907, Duncan MacDougall put dying patients on a scale.

MacDougall hoped to discover whether a soul had mass with his macabre experiment, and if so, how much. His tests with patients, later known as the 21 grams experiment, were limited in sample size and poorly designed. Nonetheless, the experiment has endured through the decades, spawning movies, books, television episodes, and recreations. Part of why MacDougall’s experiment persists is because it attempts to provide hard proof of the ephemeral. In the same way that people look for scientific proof of heaven or of an afterlife, MacDougall asked a question that has occupied philosophers and scientists for millennia. The 21 grams experiment reminds us how many of life’s mysteries remain unresolved. (Read More)