Thesis Statement

Just as the topic sentence gives the main idea of a paragraph, the thesis statement states the main idea, or the purpose, of the complete essay.  The thesis statement is usually the last sentence of the introduction.  Here is an introductory paragraph, from a student essay, with the thesis given four different ways:

TV or Not TV?

        Watching television is almost a ritual in our society.  There
        are some people who turn on the TV as soon as they get
        up in the morning.  There are some people who have to
        have the TV on while eating lunch or dinner.  There are
        people who turn on the TV as soon as they get home from
        work or school.  For most of us, watching TV is part of
        our daily routine.  Is this good for us?

Or:   This paper will consider the effect of watching so much
        TV.

Or:   However, watching so much TV makes us unhappy.

Or:  However, watching so much TV makes us unhappy
       because it makes us dissatisfied with our own lives.

Here is another example:

Plants Against Cancer

        In the past there was no hope for patients with cancer.
        All they could do was wait to die.  Fortunately, now we
        have a much better chance against cancer due to the
        identification of risk factors, advanced technologies for
        detecting cancer, and innovative therapies.  However,
        what I want to focus on here is not detection and remedies,
        but preventing cancer before it appears.  There is a low-
        tech, natural way to prevent cancer, by using plants.  How
        can plants prevent cancer?

Or:    In this paper I will consider why plants are good for preventing
        cancer.

Or:    Many plants are effective in preventing cancer because they
        contain anti-oxidants, enzymes and fiber.

Notice that these kinds of thesis statement are somewhat different:

States topic:       Is this good for us?

                        How can plants prevent cancer?

States purpose:  This paper will consider the effect of watching
                        so much TV.
                   
                        In this paper I will consider why plants are good
                        for preventing cancer,

States idea:        However, watching so much TV makes us unhappy.

States idea
more exactly:     However, watching so much TV makes us unhappy
                        because it makes us dissatisfied with our own lives.

                        Many plants are effective in preventing cancer
                        because they contain anti-oxidants, enzymes and
                        fiber.

It is easiest to state the topic with a question, or state your purpose; however, in my opinion, it is better to state your idea rather exactly.  An idea-thesis really indicates what will follow in the paper.  An exact idea-thesis also indicates what organization the reader can expect.

The thesis is the most important sentence in your paper because it controls the entire paper.  Every paragraph must relate in a clear way to the thesis.  If it doesn't your paper has lost its focus, or unity.