The paper explores definitional accounts of language structure. Several classes of arguments for definitions are reviewed, in particular those connected to classical theories of reference, theories of informal validity, theories of sentence comprehension, and theories of concept learning. It is argued that in each of these areas non-definitional accounts are preferable. A series of experimental results bearing on sentence comprehension is presented. They are said to show this for one class of examples of definitional structures, that of 'causative verbs'.