The concept of 'purposeful metaphor' is proposed as an alternative to 'deliberate metaphor' (Steen, 2008) in providing a theory of metaphor in discourse and communication. The case for 'purposeful metaphor' is framed within a discussion of intentionality in a murder trial. It is argued that 'deliberateness' originates in epistemologies based in language use, but is not valid for epistemologies that distinguish between conscious and unconscious thought process; in literary studies it is known as the 'intentional fallacy'. However, considerations of intention are relevant in critical metaphor analysis that seeks insight into the social and political motivation of metaphor. Insights from Speech Act Theory and rhetorical theory suggest that 'deliberate metaphor' could be modified to 'purposeful metaphor' because one conceptualises 'purpose' in terms of a SOURCE (or idea), a PATH (or rhetorical plan) to realise a GOAL (or rhetorical outcome). 'Purposeful metaphor' therefore integrates the source (or idea behind) path, (or rhetorical plan), and goal, (or rhetorical outcome) of metaphor, while 'deliberate metaphor' only profiles its inception. Illustrations are given of how 'purposeful metaphor' contributes to an explanation of metaphor use in political and legal discourse, and other persuasive genres. Linguistic evidence for purposefulness is in the interaction between textually complex use of metaphor and contextual features such as political purpose or describing medical conditions.