¹⁵The distinction between external and internal rewards, or internal and external motivation as they are called in psychology, may not always be very clear. Both come from the programmer or evolution anyway. See footnote 33 in Chapter 5 which discusses the definition of rewards vs. learned state-values; a similar logic has been applied on internal vs. external rewards by Singh et al. (2010); see also Doya and Uchibe (2005).