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In Logic by Wifred Hodges he says

[..] you must rely on your common sense - as always in logical analysis.

So i'm confused (again!). What has common sense to do with logical reasoning? My previous understanding is that logical was an objective discipline. So it seems odd that the subjective and vague 'common sense' is in the logical mix. Certainly with the boolean logic of computer programming that I am more familiar with common sense has no role. Well I think it doesn't but then clearly logic isn't what I thought it was.

So what role does common sense play in logic? Does it really have an element of subjectivity?

Many thanks for any/all thoughts

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    Common-sense is the root of the sciences, the arts & philosophy. Logic didn't begat logic, that would be circular. But the sense that is common to us did - as by its nature it is inate. – Mozibur Ullah May 17 '14 at 18:07
  • So, in summary, Logic will be a means of proving/disproving common sense. Like, we have a path like this? ⬇️⬇️⬇️ 1) Premise:Common sense➡️ 2) Sieve with: Tool of Logic ➡️ 3) Results:Facts. Right? And the results of arriving at an answer working with ONLY Common sense will be AN OPINION while arriving at an answer working with Logic will be A FACT. – Yemisi Oct 28 '18 at 14:20
  • The logic described by Aristotle is common-sense. He did not invent it but formalised ordinary practice. It is, after all, common sense to think 'logically' and the dialectic is what we usually use. His three laws are literally common sense. Are you sure you're right to say that common sense is not objective where logic is? I'd say this is not the case. – PeterJ Oct 29 '18 at 12:43
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I think that what Hodges means is that most of the basic principles of logic are based on our (human) linguistic and thinking habits.

Thus, the "ground" for some rules like :

"if a and b, then a"

and

"if a, then a or b"

can be found in the "usual" way we speak.

Tarski's Truth Definitions, one of the basic discovery of modern mathematical logic, can be read as a vindication of common sense view of truth : a linguistica assertion is true exactly when the facts "out there" are as the assertion says they are.

We can read Hodges' statement as a search for a "third way" between the theory about the a priori nature of logic (which is challenged by modern proliferation of "logics" : see Paraconsistent Logic and that asserting its conventional status.

  • +1 thank you for that. Apologies if this is very basic but are you saying that logic depends on (human) linguistics. So an alternative linguistic system would have different logic? I thought previous that logic was just 'out there' a priori i guess – Crab Bucket May 17 '14 at 21:36
  • @CrabBucket - logic ana mathematics are the "best candidates" to the status of a priori knowledge; still, there are some evidence against... According to some point of view, linguistic capacity is innate (and not conventional - see Chomsky); thus, we have a sort of "firmware" programmed to support us in the learning process of language. It seems that there are limits to the "possible" languages we can learn and use. If so, is there the "source" of our "logical" capabilities ? – Mauro ALLEGRANZA May 18 '14 at 7:39
  • Also linguistic capactity when seen gramatically, can be seen axiomatically. – Mozibur Ullah May 18 '14 at 15:44
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Common sense is merely non-formal, intuitive logic based on everyday experience of the real world.

The advantage of formal logic over common sense is that it can deal with proofs that are vastly more complex, and that it can deal with subject matters that are outside of everyday experiences, often yielding non- or even counterintuitive insights.

Conversely, the advantage of common sense is that it is intuitive and thus constantly and very quickly available without the need for lengthy, difficult proofs.

This makes common sense a good "sanity check" on formal logic: if your logical proof arrives at a result that contradicts common sense, then you most likely made a mistake and should double-check your proof.

Counterintuitive insights produced by formal logic are great (and often very valuable), but also not that common. They just get talked about a lot, because formally proving something that everyone already knew intuitively isn't exciting.

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One of the confusing areas is that intuitive or natural concepts of truth are based on instincts, neurons, and nerves in us living creatures; these are extremely ancient in origin and are not at exactly the same as modern Boolean and machine logic.

For one thing, nerves and neurons, at least as ancient as octopuses and squids, have several different ways of estimating that something may be true and other, also different, ways of determining that in fact something is always true or always false or at least true (or false) for the moment.

Machine logic, including both the fundamental bits and bytes and the organized complexities in machine learning (such as the phenomenally successful Alpha series of computers) is essentially bit by bit, de rigeur and has little or no anticipatory or intuitive component at all. Yet, anyway.

It is thus good to allow a dual, orthogonal dimension of independence between natural logic and mathematical/machine logic.

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Wikipedia describes common sense as...

sound practical judgment concerning everyday matters, or a basic ability to perceive, understand, and judge that is shared by ("common to") nearly all people.

Albert Einstein argued that...

common sense is the collection of prejudices acquired by age eighteen.

Rebecca Stead pointed out that...

Einstein says common sense is just habit of thought. It's how we're used to thinking about things, but a lot of the time it just gets in the way.

I happen to agree with that. Common sense is basicly a variety of prejudices that are useful in assessing daily life. And it allows you to distinguish between fact and folly quite easily, as long as you stick to everyday matters.

However, Common sense becomes useless once you go beyond everyday matters. And this is where the scientific method comes in. The further you move away from everyday matters, the more useless common sense becomes and the more necessary the scientific method becomes to anyone who cares about assessing a situation.

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