The article explores the possibility that studies in cerebral commissurotomy (severing of the corpus callosum) may shed some light on Jean-Paul Sartre's account of bad faith. I examine this issue from both a descriptive and an explanatory point of view. My conclusion is that Sartre and various neuroscientists seem generally to agree on the description of self-deception, but they substantially disagree on how the phenomenon is to be explained. I argue that Sartre's account does not seem fully adequate, while the neuroscientific approach may have explanatory potential if certain conceptual issues are resolved. I also argue that, carefully delimited, the neuroscientific stance complements Sartre's approach.