When a child wants to do something forbidden, often you hear an argument that goes something like this:
Child: “There’s nothing wrong with getting a tattoo. Other kids my age do it all the time.”         Â
A common reply by parents quickly identifies the hidden assumption in the child’s argument:
Parent: “And if other kids jumped off a cliff, would you do that too?”
The parent quickly sees that the child’s argument is based on a faulty, underlying assumption: If an action is done frequently by others, nothing is wrong with it. Some of the most important arguments in today’s society also have underlying assumptions that need to be made explicit in order for productive discussion to take place. For example:
Thesis: “Abortion is wrong because killing an innocent person is wrong.”
The assumption here is that abortion involves the killing of a “person.” Indeed, defining exactly when a human egg, embryo or fetus attains “personhood” is a key part of the  abortion debate and of any paper written with this thesis. For our writing to be sound, we must deal with these underlying assumptions. In writing, all of our assumptions should be clear and supported. For example, let’s say this is your thesis for a paper:
Thesis:Â Teachers should not fail students because failing grades damage a student's self-confidence.
The underlying assumption is: Teachers have a duty not to damage students' self-confidence. Teachers may or may not have this obligation. That’s up to the writer to argue successfully. The point is this: For this thesis or any paper based on it to be successful, the writer must be aware of its underlying assumptions, make those assumptions explicit, and provide adequate support for them in order to be convincing.