An assumption, for the purposes of A-level Critical Thinking, is an unstated reason. It is something that must be true for an argument to work, but which is not explicitly stated in the argument.

For example, the argument “The college address is the same street as I’m standing on; therefore, the college must be nearby” assumes that the street isn’t very long. If the street is long, then the college could be on it but still miles away.

The Negative Test

To test whether something is assumed by an argument, you can use the negative test. This involves inserting the opposite of the alleged assumption into the argument and seeing if it still makes sense.

For example, to test whether the above argument assumes that the street isn’t very long, we would check whether this makes sense: “The college address is the same street as I’m standing on; the street is very long; therefore, the college must be nearby.”

With the opposite of the alleged assumption inserted, the argument clearly doesn’t make sense, so the alleged assumption must be true for the argument to work; it is assumed by the original argument.

Don’t Quote!

When answering a question that asks you to identify an assumption, unlike when answering other questions, you should never give a quote from the text; by definition, assumptions are unstated.