A little studied, but essential aspect of human natural-language production is the ability to form concise descriptive expressions.... A very simple model of sentence production involves two steps first one has a thought, and then a sentence is chosen out of an infinite number of possibilities which expresses the thought. For example, a formal semantic model can be given in which the same 'thought' (an expression in first-order predicate calculus) is expressed by the following four sentences I see the big red thing I see the thing that both big and red It is the thing which is red and which is big I see What I here and now see is the thing which is big and not small and that is either round or not round and which has the property of being red.