His major areas of interest include moral philosophy, philosophy of mind and action, and 17th-century rationalism. His 1986 paper "On Bullshit", a philosophical investigation of the concept of "bullshit", was republished as a book in 2005 and became a surprise bestseller, leading to media appearances such as Jon Stewart's "The Daily Show". In this work he explains how bullshitting is different from lying, in that it is an act that has no regard for the truth. He argues that “It is impossible for someone to lie unless he thinks he knows the truth. Producing bullshit requires no such conviction.” In 2006 he released a companion book, "On Truth", which explores society's loss of appreciation for truth.