 George Lakoff and Mark Johnson, although originally taking a hard-line interpretation of these early authors later concede that Aristotle was working within a different philosophical framework from what we engage with today and that critical interpretations should take this in to account.In his 2007 book "The Stuff of Thought", cognitive scientist Stephen Pinker lays out several useful classifications for the study of conceptual metaphor.Pinker first contrasts two perspectives on metaphor, what he calls the killjoy theory and the messianic theory.