John and Ken begin by wondering how thoughts are represented in the brain--are there words, sentences, and phrases that represent what we often use words, sentences, and phrases to describe? Or are there pictures or symbols whose relationships we store and can retrieve during cognition? John discusses the history of this debate with examples from Descartes, and Ken points out why, for the most part, these ideas were rejected in favor of more linguistic solutions in the twentieth century. Ken argues that mental images must resemble our perceptions in order to make sense, but that we have no possible way of knowing what is really out there. John points out that having "mental words" rather than "mental pictures" doesn't really solve that problem, but he concedes that there is a certain circularity inherent in the mental picture representations.

Ken introduces Lera Boroditsky, Assistant Professor of Cognitive Psychology at Stanford University, and John asks her about the debate between scientists who believe we represent thoughts using imagery and those that think we use some other kind of system. Ken jokingly asks why scientists need brain imaging for these kind of studies instead of simple philosophical introspection, and Lera points out that some people report that they think in images, and some people claim they think in words, so simple introspection is not definitive. John then tries to get at the real definition of mental imagery, since philosophers have interpreted this concept differently. Lera talks about how over the course of the debate the idea of mental images has been refined, and gives an interesting example concerning a penny and the level of detail in mental images.

In order to clarify the debate, Lera frames it as an argument over the currency of thought: what are thoughts made of on a fundamental level? There are many different views, some say that there are only symbolic representations, some claim there are solely mental images, and other, more reasonable scientists argue for a combination of the two types, although the degree to which one type dominates the other is still vigorously debated. John talks about the use of maps and directions as an analogy for brain representations, and Lera points out that people operate very differently when given maps or verbal directions. Ken asks Lera to discuss why she believes human beings use mental images, and Lera explains the scientific evidence that we use mental images and that they in fact preserve many aspects of real images like size, structural relationships, and can be transformed into other new images through rotation and combination.

John, Ken, and Lera move on to discuss different kinds of mental imagery, as well as examples of mental imagery and mental symbols, and callers put in their opinions and relate their experiences about using mental imagery in different situations. John, Ken, and Lera conclude by discussing imagination and dreams and the roles that language and imagery play in those phenomena.

• The Roving Philosophical Reporter (Seek to 4:23): Zoe Corneli discusses mental imagery and mental images with renowned Harvard Psychology Professor Stephen Kosslyn, one of the key figures in the mental image vs. mental symbol debate in cognitive neuroscience.
• Conundrum about Obligations after Betrayal (Seek to 47:56): John and Ken try to help someone decide what her obligations are to someone who hurt and betrayed a close friend and also happened to loan her some nice speakers. Should she give them back or even tell him she doesn't want them anymore? Do his later actions cancel out her prior obligations?