All arguments, whether inductive or deductive, begin with assumptions (also known as premises). An argument may be deductively valid (that is, if its assumptions are true then its conclusion must be true) but rendered irrelevant or unhelpful in practice by unacceptable assumptions. For example:
The Pope is a secret Hindu.
If the Pope is a secret Hindu, he can’t really be a Catholic.
Therefore, the Pope isn’t really Catholic.
This is all entirely valid: if the assumptions are correct, then the conclusion is too. However, the assumption that the Pope is a secret Hindu is, to say the least, implausible. An argument is only as good as its assumptions, and it’s important not to be seduced by the slickness or the complexity of an argument into taking its assumptions for granted. Unfortunately there are a great many philosophy books, for example, that I’m confident are not worth spending time reading because, although their arguments are rigorous, they are based on unacceptable (and to my mind, insufficiently examined) assumptions.
Assumption-spotting is perhaps the most crucial practical skill in Critical Thinking. The key issue here is whether what you think may be an assumption is actually necessary to the argument. If you assume that someone is making an assumption that they are not making, then obviously this is unfair. If an assumption is present, then it would have to be true for the conclusion to be true. For example:
John is out.
His coat is missing from the peg.
Here, it is being assumed that John must take his coat when he goes out. It is also being assumed that he only has one coat, and that the coat is not missing because someone has stolen it, or for some other reason.
However, if someone were to claim that “John is a man” was an assumption here, this would be incorrect. John does not have to be a man for the conclusion to be correct. John could be an alien or a polar bear and the conclusion would still be fine.
Assumptions can be classified into explicit, implicit, and background types. An explicit assumption is stated in the argument: in the above example, this is “His coat is missing from the peg”. We assume the accuracy of this information in the argument. Implicit assumptions are not stated, but nevertheless must be true for the conclusion to be true. So, in the above example “John must take his coat when he goes out” is an implicit assumption required to reach the conclusion. “John has only one coat” is also an implicit assumption, but of a kind we would call a background assumption. It has to be correct for the conclusion to be correct, but it doesn’t play a direct role in the reasoning. Rather, it is taken for granted.
The distinction between foreground and background assumptions is not hard and fast – it will vary with the context. Some more sceptical types will be more inclined to question background assumptions than others! However, it is helpful to recognise that some assumptions are more immediately important in the context than others. “The universe exists” is a background assumption in almost all arguments, but not one we need bother discussing most of the time.
Exercise
Identify the assumptions in these arguments. If possible, distinguish between explicit, implicit and background assumptions.
1. The upper decks of double-decker buses are best avoided. I’ve often found them to be full of rude teenagers playing loud music with no concern for the feelings of other passengers.
2. The 2003 invasion of Iraq was illegal because it was motivated only by a desire for oil.
3. Naomi had better watch out! There’s a polar bear behind her!
4. Since there has been no snow in southern England so far this winter and it’s now the middle Of January, we can conclude that it will be a mild winter throughout the UK.
5. Richard III’s body was found under a car park in Leicester, and he fell nearby in Bosworth Field, so it is only right that he should be re-buried in Leicester.
Picture: Pope Benedict XVI by Agencia Brasil (Wikimedia Commons)