Internal state predicates or ISPs refer to internal states of sentient beings, such as emotions, sensations and thought processes. Japanese ISPs with zero pronouns exhibit the "person restriction" in that the zero form of their subjects must be first person at the utterance time. This paper examines the person restriction of ISPs in Japanese in contrast with those in Thai, which is a zero pronominal language like Japanese. It is found that the person restriction is applicable to Japanese ISPs but not to Thai ones. This paper argues that the person restriction is not adequate to account for Japanese and Thai ISPs. We propose a new constraint to account for this phenomenon, i.e., the Experiencer-Conceptualizer Identity (ECI) Constraint, which states that "The experiencer of the situation/event must be identical with the conceptualizer of that situation/event." It is argued that both languages conventionalize the ECI constraint in ISP expressions but differ in how the ECI constraint is conventionalized.