Thinking clearly requires us to locate any hidden assumptions in the argument before us. Unstated claims are part of any discussion, and identifying the assumptions is key to evaluating the quality of an argument. What makes hidden assumptions so difficult to spot is that large numbers of people often share the same unsupported, unstated assumptions. To find a hidden assumption, we have to find “the shift.”
Finding the shift involves two key steps: first, locating the premise and the conclusion, and then examining the gap between the two. That gap is “the shift.” Once you have the basic structure identified you can look at how well the conclusion flows from the claim. Is it a smooth shift, or are we grinding the gears? How big is the gap between the assumption and the reality? Let’s look at an easy example:
My bank closes at 3pm on Mondays, and it’s 5:30pm right now. My bank is closed.
Identify the premise and conclusion, then find the shift. What must be present in the argument to make the conclusion true? In this case, it must be Monday. If it’s actually Friday, then the assumption (that it is Monday) must be false. We must stipulate that today is Monday for the claim to be credible. Here’s an example that’s a little more challenging:
A new form of cancer that kills only people who are left-handed has been diagnosed in my brother-in-law. My brother-in-law’s life is at stake and we must begin treatment immediately.
Identify the premise and conclusion, then find the shift. What is the unstated premise that must be true in order for the conclusion to be true? In this case, there are a couple. One, that my brother-in-law is left-handed and, two, that immediate treatment will save his life. If my brother-in-law is right-handed and the cancer kills only those who are left-handed, then the assumption that my brother-in-law’s life is at stake is false. The other assumed premise—that immediate treatment is necessary—is also false. Past medical practice has shown that many cancers can be treated successfully over an extended time period, so the assumption that cancer is a medical emergency requiring immediate treatment is also false. If the assumptions are false, then so are any claims built on those assumptions.
Using these steps allows you to spot the weaknesses in the argument. If the argument is flawed, you’ll know why. Mastering this one skill—identifying assumptions—will dramatically improve your critical thinking skils.