The book centers on the concept of the memory palace, a mnemonic device that dates to antiquity. The concept was quite popular in the Renaissance, especially in Counter-Reformation Catholic culture. The book is premised on Ricci’s attempt to convert people preparing for Chinese administrative examinations in the Ming Dynasty to Catholicism by teaching them an effective mnemonic device under the belief that its success would demonstrate the superiority of Catholic teachings.

The memory palace is rooted in the method of loci, which is based on the recognition that people are good at remembering locations. By associating abstract concepts or unfamiliar information with a well-known place, memory recall becomes easier. In the memory palace, the method of loci is applied to construct a place—real or entirely fictive—that can hold everything you need to remember. The place can be a city, a house, or a room. Information is stored as an image, which is placed into the space until you need to recall it. The place should be quite familiar to the individual, and the location should be detailed. Roman rhetorician and educator Marcus Fabius Quintilian described how one would store images in a place, saying,