An argumentative or persuasive piece of writing must begin with a debatable thesis or claim. In other words, the thesis must be something that people could reasonably have differing opinions on. If your thesis is something that is generally agreed upon or accepted as fact then there is no reason to try to persuade people.
Example of a non-debatable thesis statement: Pollution is bad for the environment.
This thesis statement is not debatable. First, the word pollution means that something is bad or negative in some way. Further, all studies agree that pollution is a problem; they simply disagree on the impact it will have or the scope of the problem. No one could reasonably argue that pollution is good.
Example of a debatable thesis statement: At least 25 percent of the federal budget should be spent on limiting pollution.
This is an example of a debatable thesis because reasonable people could disagree with it. Some people might think that this is how we should spend the nation's money. Others might feel that we should be spending more money on education. Still others could argue that corporations, not the government, should be paying to limit pollution.