Defining Your Thesis
Like topic sentences, theses can be simple (stated explicitly, either in one sentence or in several consecutive sentences), delayed-completion (begun in one sentence and completed at some point later in the essay), assembled (scattered in bits and pieces throughout the essay), or inferred (never explicitly stated--left for the reader to surmise) (Braddock, 310-323). But however the thesis is presented, it should be clearly defined, or, in the case of an inferred thesis, clearly definable. Even if you have chosen to use a delayed-completion, assembled, or inferred thesis, you should be able to articulate that thesis in a simple, explicit statement.

Two things happen when you fail to define your thesis clearly:

First, you don''''t know what you have committed yourself to--in fact, you may not have committed yourself to anything. As a result, your paper lacks unity. A unified essay is one in which all of your arguments, directly or indirectly, support your thesis. (Although good writers do acknowledge opposing points of view and may even concede a point here or there, they usually do so for rhetorical purposes--to enhance their own credibility by indicating that they are aware of and capable of responding to opposing views.) If you have not defined your thesis clearly, you will not know what your arguments should support. Consequently, you will ramble: some of your arguments will be irrelevant to any thesis your readers might infer; others will be contradictory. Whatever unity you achieve will be largely accidental.
The second consequence of an inadequately defined thesis stems directly from the first: when you don''''t know what you have committed yourself to, your essay lacks unity, and your readers have no thread to help them find their way through your thoughts. As you ramble, your readers grope.
Focusing Your Thesis
A thesis can be clearly defined and still lead to a rambling essay if it is not adequately focused. A good thesis narrows your topic to an idea that you can successfully develop within the framework of your essay. From the general topic of health hazards, you might propose a thesis such as, "The average American is exposed to many health hazards." This thesis, though clearly defined, is so broad that you would never be able to cover it adequately in a short essay. You would wind up either jumping from one health hazard to another, discussing each only superficially, or zeroing in on one or two health hazards and, thus, failing to demonstrate your own thesis. A more narrowly focused thesis, such as "The Constitution of the United States should be amended to prohibit the production and sale of cigarettes," commits you to an idea that you can carefully analyze and defend in four or five pages.