1) Supporting Your Argument 

Most arguments are built on assertions—statements that you make about a debatable topic—backed by evidence—supporting information, in the form of examples, statistics, or expert opinion. If, for instance, you asserted that law-enforcement officials are winning the war against violent crime, you could then support this assertion by referring to a government report stating that violent crime—especially murder—has dramatically decreased during the past decade. This report would be one piece of persuasive evidence. 

Only assertions that are self-evident (“All human beings are mortal”), true by definition (2 + 2 = 4), or factual (“The Atlantic Ocean separates England and the United States”) need no proof. All other kinds of assertions require support. 

NOTE: Remember that you can never prove a thesis conclusively—if you did, there would be no argument. The best you can do is to provide enough evidence to establish a high probability that your thesis is reasonable or valid.