This chapter discusses the private-language argument, private experience of bodily feeling, ethology and natural expressions, the substitution principle, primary and secondary language-games, physiognomic language-games, expressive versus descriptive language and the asymmetry principle and the beetle in the box simile. It also includes grammatical remark, numerical and qualitative identity, applications to color words and personal identity. The 'private-language argument' (PLA) employs a complex web of analyses and arguments sparked by the question of whether a single individual could create a language by associating words with his or her own personal and private experiences. In considering the communication and understanding of private experiences of bodily feeling, Wittgenstein reveals two common presuppositions, neither of which is defensible when brought to light. In the seventeenth and eighteenth centuries philosopher-psychologists tended to think of the contents of the mind as 'ideas,' or mental entities that are in many respects like material things.