Unity and Emphasis in the Expository Paragraph

The law which our descriptive models should have impressed upon us in regard to the first, the last, and the intervening sentences in a para¬graph, applies also to this new type of paragraph ; that is, the first sentence should express a general truth or make a statement about either a class of objects, an abstract idea, or a general process, the sentences following should explain this statement, and the last sentence should summarize or give the reader a sense of completeness.