Critical Thinking 3: Assumptions

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.Pope Benedict XVI Agencia Brasil

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.


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)

About Robert M Ellis

Robert M Ellis is the founder and chair of the Middle Way Society, and author of a number of books on Middle Way Philosophy, including the introductory 'Migglism' and the more in-depth 'Middle Way Philosophy' series. He has a Christian background, and about 20 years' past experience of practising Buddhism, but it was his Ph.D. studies in Philosophy that set him on the track of developing a systematic account of the Middle Way beyond any specific tradition. He has earned his living mainly by teaching, and more recently by online tutoring.

12 thoughts on “Critical Thinking 3: Assumptions

  1. 1. In this argument, the conclusion that the upper decks of buses be avoided uses the passive mood, delinking the personal experience of the proposer’s discomfiture (an implicit assumption that everyone shares it) from the conclusion’s universal application.

    2. In this argument, the conclusion on the legality of war is based on an implicit assumption that wanting oil is against the law.

    3. In this argument, the conclusion that Naomi ought to be wary is based on Naomi’s not being able to see what’s behind her, and the highly improbable background assumption that polar bears are commonplace visitors.

    4. In this argument, the conclusion contains an explicit assumption that an absence of winter snow in southern England is a reliable indicator of a mild winter weather more widely. This seems a reasonable prognostication, given that background assumptions on the predictability of weather tend to weak.

    5. In this argument, a conclusion on burial rights contains explicit assumptions that those rights are determined by the place of death, and where the body is found.

    1. 1. I think the first question is an implicit assumption but the argument is implausible, it depends on the time of day for example, the teenagers could be in school.
      2. Here an explicit argument is made, the war is illegal, but the conclusion (it is illegal) may be true or not true, other factors may have been in play, regime change for example.
      3. Here is a background (general) assumption, I think the assumption is irrelevant, polar bears can be dangerous, but who would allow a person to be close to a bear without taking precautions?
      4. This is an explicit assumption, ‘there is no snow therefore it won’t snow’ but the conclusion of the argument is implausible, the vagaries of British weather!
      5. This is an explicit assumption, ‘his body was found’ the conclusion or reason to keep his body there is implausible, by being a royal person many other factors are considered.

  2. 3. There’s an implicit assumption that, because Naomi has her back to the bear and can’t see it, she doesn’t know it’s there. Maybe she can hear it, smell it, or uses female intuition…..:) There’s a background assumption that Naomi is a human female, but she might be a sabre-toothed tiger or a wooly mammoth. There’s an implicit assumption that ‘behind her’ means in immediate proximity to her; but the polar bear might be many miles behind her.

    You can tell I’ve acted in pantomime, Norma. “Oh no you’ve not! Oh yes I have…..”

    1. Hi Peter, I like your pantomime phrase. Oh yes I must admit it doesn’t take much to make me have a rethink. So, ‘There’s a polar bear behind her’ is an implicit assumption of danger, would the conclusion that she should watch out be true and plausible – probably. I’m really not sure!

      1. The argument is couched in language, as I see it, that is deliberately constructed for pantomime effect. It is stuffed with absurdities, it includes several assumptions (explicit, implicit and background ones) that – taken together cause such dissonance that they make on either scratch one’s head, or smile a crooked smile. In a pantomime situation, where one is already sensitised to the absurd, and disinhibited by having one’s thinking derailed, it would raise a laugh.

        At least, that’s my crackpot theory…….

  3. Hi Robert, Thank you for the video, it is so true what James Randi says in such an amusing way, we have come to trust that certain actions we make will be safe. In order to function smoothly it seems that we have to be confidently trusting.
    Thank you Peter, it is fun when our thoughts spin off at a tangent and land in new territory.

  4. 1. The implicit assumption here is that the other people would also want to avoid using the upper deck due to the circumstances described in the implicit assumpion. “I’ve often found them to be full of rude teenagers playing loud music with no concern for the feelings of other passengers” . There is another (background ) assumption that people would believe the frequency and accuracy of the explicit assumption.
    2. The implicit assumption here is that wars are illegal if only motivated by desire for a natural resource such as oil. A background assumption is that wars should be legal.
    3. There are implicit assumptions here that Naomi is a human being, that she is not visually impaired, that the polar bear is alive, not in a cage, dangerous or that Naomi doesn’t have a death wish.
    4. The assumption here is that because the winter has been mild up to the middle of January, it will continue to be so. A background assumption is that England has a climate where snow is a likely occurrence in winter.
    5. An implicit assumption here is that bodies should be buried near where they have been found. A background assumption is that the body should be buried and not cremated, frozen or put in a museum.

  5. Here are my answers:

    1. Explicit assumption: the second sentence
    Implicit assumption: Places with rude teenagers should be avoided

    2. Explicit assumption: The invasion was motivated by a desire for oil
    Implicit assumption: Being motivated by a desire for oil makes an invasion illegal. (This doesn’t necessarily have to be generalised to other natural resources, Barry).
    In context, Barry’s probably right that there’s a background assumption that wars should be legal: however, it isn’t actually stated that the war was wrong.
    Norma – whether or not the conclusion is actually true isn’t relevant to what the assumptions are. It’s just about what would hypothetically have to be true for the conclusion to be true.

    3. Explicit assumption: There’s a polar bear behind Naomi.
    Implicit assumption: The polar bear provides a reason for Naomi needing to watch out (i.e. it is dangerous).
    There are loads of background assumptions here, some of which Barry and Peter spotted: e.g. that the polar bear is alive, close behind, awake, not in a cage etc; also that Naomi doesn’t have a death wish, yes, or that Naomi is not protected by a cage or other barriers.
    Some of the background assumptions suggested do not have to be the case for the conclusion to be correct. Naomi doesn’t necessarily have to be human : she just has to be vulnerable to polar bears. If she was a rabbit this would still be the case. It doesn’t make any difference whether or not Naomi can see the polar bear if “watch out” just means “be on your guard”. We also don’t have to assume that polar bears are commonplace.

    4. Explicit assumptions: The first part of the sentence up to the comma
    Implicit assumptions: That the mildness of the weather so far will continue for the rest of the winter. That the weather in Southern England is a guide to the whole UK. Barry’s also right about the background assumption.

    5. Explicit assumptions: The first two phrases.
    Implicit assumption: That where the body is found and where it originally died should determine where it is re-buried. A background assumption here is that there are no more important considerations that overrule this criterion.

  6. Hi,

    I am a week behind, but I’ll have a go anyway.

    1. There will be a lesser amount of rowdy teenagers on the lower deck of the bus. (I think that this is an implicit argument – possibly background, but I am not sure).

    2. A war motivated only by a desire for oil is illegal (I think that this is explicit).

    3. Polar bears are dangerous (implicit – changed from explicit).

    4. The weather in southern England can be used to make predictions about the weather throughout the UK (Implicit – I think)?

    5. Kings should be buried near where they have died. (Implicit).

    That was tough!


    1. Hi Rich,
      1. The lesser amount of teenagers on the lower deck is a background assumption – one I failed to note above.
      2. The war being illegal is the conclusion of the argument, not an assumption.

      I’d agree with your other answers. More details in my post above.

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