Jackendoff (1997), whose analysis of idioms is based, in part, on work by Nunberg, Sag & Wasow (1994), discusses VP idioms and addresses the question of mobility. Both works identify fixed idioms, such as kick the bucket, and mobile idioms, such as spill the beans and take advantage of. Fixed idioms are ones whose NP objects are impervious to syntactic operations, as illustrated by the unacceptability, in their idiomatic sense, of sentences like *The bucket was kicked by Bill; while mobile idioms occur in sentences like The beans were spilled by Fred and Advantage was taken of Bill. Jackendoff correlates the mobility of VP idioms with a property that he refers to as metaphorical semantic composition. However, he observes that this property is not a sufficient condition for mobility.

I will argue that the property of metaphorical semantic composition be replaced by a property of thematic composition, and that this property is a sufficient condition for mobility. A closer inspection of mobile idioms that have thematic composition reveals that they fall into two subtypes: expressions that have a property of ‘transparency of interpretation’, and ones that do not have this property. I refer to members of the first subtype as METAPHORS. I will demonstrate that there are no idiosyncratic constraints on their syntactic mobility, and will conclude that they need not be encoded in lexical entries as phrasal idioms. In these respects, they are distinct from members of the second subtype, whose degree of mobility is more limited, and which must be encoded in lexical entries as phrasal idioms. Finally, I will address the question of the necessity of thematic composition for mobility. Throughout the paper, I will assume that phrasal idioms are appropriately encoded in lexical entries of the types proposed by Jackendoff for fixed and mobile expressions.