Experiencer is a thematic relation associated with noun phrases whose referent is capable of experiencing senses or emotions. For example, the subject of the sense verbs "smell, see, taste, hear, look, feel, listen, the emotion predicates like, hate, endure, love as well as predicates of mental state like understand, think, believe, disbelieve, etc. are all experiencers. For example, in the sentence Bill loves Pat, Bill is the experiencer.
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Carnie, A. (2013) Syntax: A Generative Introduction. 3rd Edition. Wiley-Blackwell