In the middle of last semester, I had an interesting conversation with another writing consultant. The topic of our conversation was the difference between an assumption and a presumption. Although when I check The Shorter Oxford English Dictionary, it does, in the second entry, give assumption as a synonym for presumption, when philosophers make arguments, assumption is the term of choice.
Most of you probably have a colloquial sense of what an assumption is, indeed, most of you have probably been warned about the dangers of assuming anything. When you make an argument or when you assess an argument, you should probably avoid relying on your colloquial intuitions.
Assumptions can be true or false; they can be strong or weak. They will be found in almost every argument you will ever encounter. Thus, while making an ass of oneself and others, could on many occasions result from making a naive assumption, it certainly is not inevitable.
What is an assumption?
This was the question put to me by the other writing consultant. After thinking back to my Logic 101 class, my response was the following: "An assumption is an unstated premise." Fairly simple, right?
We know that in college classrooms "argument" does not mean "screaming match" or "fight." Rather, when you make an argument for a class, you conclude something on the basis of evidence and good reasoning. The premises are used to support the conclusion. Sometimes, if the argument is sufficiently complex, you may have more than one conclusion. In such arguments, you may have a sub-conclusion that also a premise because it is used to support a main conclusion. So, premises support a conclusion, and an assumption is an unstated premise.
To illustrate these abstractions, I'm going to refer to some of what I said to my writing consultant colleague. Before our discussion began, we had just noticed someone eating chicken soup in the graduate student office, at which point I said, "He must have a cold." My colleague confirmed my conclusion. "Yeah, he told earlier today that he wasn't feeling well."
This exchange, or an exchange very similar to this one, happens a million times a day. But what exactly are the rhetorical components that are important for critical thinking? There are premises and a conclusion, and of course, there are assumptions as well. To make the relationship between these entities explicit, let's look at the following schema.
Conclusion: "He must have a cold."
How did I arrive at this conclusion? Well, I saw the person in question eating a bowl of chicken soup. So let's make this premise explicit.
Premise 1: "He is eating a bowl of chicken soup."
At this point, something in the reasoning chain is missing. How do I connect premise 1 to the conclusion? There are many ways, but the most definitive would be to use the following premise.
Premise 2: "If a person is eating a bowl of chicken soup, then he has a cold."
Premise 2 is false, but it will help illustrate my point about assumptions. The point is that I didn't actually include another premise when I drew my conclusion. My argument, if I had written it down in standard premise and conclusion form would have looked like this:
1. He is eating a bowl of chicken soup.
2. Thus, he must have a cold.
The consultant I was speaking with understood my reasoning chain without the additional premise. When I am speaking to someone in a colloquial manner, it would be awkward, not to mention cognitively problematic, to explicitly state every premise in every argument I make. For this reason, I, and basically everyone else in the world, will use a lot of assumptions when we argue. Let's include the assumption in our schema to reinforce this idea visually.
1. He is eating a bowl of chicken soup.
Assumption, unstated premise: If a person is eating a bowl of chicken soup, then he has a cold
3. Thus, he has a cold.
In many of the texts you read for your classes, the assumptions will be more controversial. Controversy can lead to emotion, and emotion can lead to a person wanting to make an argument appear more cogent than it actually is.
If the writing consultant I was talking to had asked me to clarify my assumption, and if I had clarified it with premise 2, then my conclusion would not follow. I would need to modify my assumption in order to get a cogent reasoning chain. I might modify my assumption by making it more narrow e.g., "If a person is eating a bowl of chicken soup, then it is likely he has a cold."
I might modify it by including more information in it e.g., "If a person is eating a bowl of chicken soup during the cold and flu season in Charleston, IL, then it is likely that he has a cold."
An essential part of critical thinking is to ask yourself about the assumptions a writer or speaker makes. If a writer's or speaker's assumption is weak, then his conclusion will be weak. Keep this in mind when you analyze an argument.