We have as yet no very satisfactory definition of sentence. The school definition: 'A sentence is the expression of a complete thought' is very nearly meaningless, since it is impossible to tell whether thoughts are complete or not. Perhaps the best available formula, despite its roundabout wording, is that of Bloomfield (1933):
'When a linguistic form appears as part of a larger form, it is said to be in included position; otherwise it is said to be in absolute position and to constitute a sentence.' Thus, ate the rat does not normally stand in absolute position, though in special circumstances perhaps it might; such a form occurs normally in 'included position' in such a sequence as 'Bonzo ate the rat', which is a sentence because of its capacity to stand alone. The basic criterion for a sentence is thus probably that of being grammatically selfcontained. We can describe it provisionally, as a minimum complete utterance.