http://sp.dictionary.com/dictstatic/d/g/speaker.swf [uh-sur-shuhn] Show IPA
1.a positive statement or declaration, often without support orreason: a mere assertion; an unwarranted assertion.– dictionary.com
Substantiate what you say with logical explanations, backed by evidence and examples.
As you write, imagine there’s an annoying sceptic in your head that keeps going, “Really? How do you know? Is that true? Why is it true?”. By the end of your paragraph, you must have been convincing enough to convince the most sceptical of sceptics.