# What is logic?



## BlueWolf

http://forum.wordreference.com/showpost.php?p=1786417&postcount=49
_
Let's have a candid conversation based on logic, not on religion or some other irrational belief system._

Since this forum is full of so logical people, I'd like some of them explain me what is logical in their point of view.
What is your Truth, so that you can define something as logical/right? I'd like to understand, because when I hear these things, I always see people that say nothing can be considered objectivly true and right, but they all follow the "logic". Introduce me it, please, because I really have some problems to understand what it is.


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## Ana Raquel

Logic deals with reason, reason with understanding,truth deals with proofs and relationships.


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## cuchuflete

Logical and irrational belief systems are not contrary.  They exist on different planes.  

Here are a few dozen definitions:

click


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## Hockey13

Logical thought processes lead me to agnosticism. I do not make a leap of logic at any step of the way (that I know of) and find myself firmly planted in the "I haven't got a clue" seat. Without a leap of logic in either direction, we only know what has been scientifically proven. If we stand by any other standards, just who are we trying to fool??

On a separate note, how's it going, BlueWolf?


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## modus.irrealis

For me, a logical argument is one where if the premises are true, then the conclusion must be true, so if someone agrees with the premises but denies the conclusion, they're being illogical. All other arguments are non-logical, but not illogical, e.g. scientific arguments are usually non-logical since even if an experiment is done a million times and gives the same result, it's always possible that the millionth-and-first time, it will give a different result, so you can't conclude that the result will always be true. You obviously then get into arguments that show that something is more or less likely, but since the conclusion is not necessary, for me, it's not a logical argument.

Of course, if two people don't agree on the premises, then logical arguments don't help out, since you can't use logic to evaluate whether the premises are true or not, unless you prove those premises on the basis of some other premises, but this goes on forever and at some point you need to stop and say this premise cannot be proven to be true and just has to be accepted as such (usually this will just be those beliefs that the people discussing will share).

Although, I've noticed that "logical" often has a more extensive meaning where it just means a "good" or "rational" argument, but these seem to be very subjective concepts, so I don't know what to make of that.


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## BlueWolf

Hockey13 said:


> On a separate note, how's it going, BlueWolf?



I'll PM you. 



modus.irrealis said:


> Of course, if two people don't agree on the premises, then logical arguments don't help out



Yeah, that's what I think too, for this reason I find these comments and this constrast faith/reason quite illogical.


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## Redisca

Logic (as opposed to ethos or pathos) is merely a mechanism for arriving at a conclusion.  The assumptions on which the argument is premised, however, can be derived from religion or another "irrational belief system".  Even religious fanatics, racists, homophobes, etc. can argue logically.  However, certain assumptions are so deeply ingrained in our culture that we take them for granted without realizing that they are arbitrary.


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## Cnaeius

Redisca said:


> Logic (as opposed to ethos or pathos) is merely a mechanism for arriving at a conclusion. The assumptions on which the argument is premised, however, can be derived from religion or another "irrational belief system". Even religious fanatics, racists, homophobes, etc. can argue logically. However, certain assumptions are so deeply ingrained in our culture that we take them for granted without realizing that they are arbitrary.


 
The point is just this: how do you state that an assumption is arbitrary or not? 
For some people it sufficient to say that if a thing comes from religion it is arbitrary, but it is a pathetic and wrong sillogism
I would tell more: certain assumptions coming from religion are so ingrained in our culture that we forget they come from a religion or other cultural element that can be irrational, most of all if they seem logic and rational.
We have a lot of history behind us. It would be a pity if we forgot all in order to be more "logic"


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## Redisca

Cnaeius said:


> The point is just this: how do you state that an assumption is arbitrary or not?
> For some people it sufficient to say that if a thing comes from religion it is arbitrary, but it is a pathetic and wrong sillogism


You are right.  The characterization of "logical" or "rational" (and otherwise) is inapplicable to assumptions; rather, such characterizations only describe the process of arriving from assumptions to conclusions.  That is why, the point I was trying to make is that one cannot juxtapose logic with religion or another "irrational system", since logic can be applied within such systems as well.  

We could assume however (darn!  another assumption), that "arbitrary" is anything reached through public concensus (e.g. "murder is wrong"), whereas "non-arbitrary" is anything that is readily observable and does not lend itself to substantial dispute (e.g. "water expands when it freezes").


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## maxiogee

I would suggest that arbitrary is anything which involves a judgement - the best x, the proper y, the right z.

We wish to buy curtains for a room - arbitrary is my wife's concern that they be of such a fabric and design so as to match other items in the room - non-arbitrary is my concern that they be of whatever fabric will provide the best insulation possible as the room is prone to coldness and draughts.
Logic dictates I should get my way, experience teaches me not to hold my breath!


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## BlueWolf

Redisca said:


> We could assume however (darn!  another assumption), that "arbitrary" is anything reached through public concensus (e.g. "murder is wrong"), whereas "non-arbitrary" is anything that is readily observable and does not lend itself to substantial dispute (e.g. "water expands when it freezes").



It isn't so simple however. Even simple readily observable events aren't sure. All these affirmations are now supposed as wrong:
-Heavy objects fall more rapidly than light ones.
-Time is something that is the same everywhere.
-The Sun runs through the sky around the Earth.
That does seem strange, since they don't "lend themself to substantial dispute".


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## ireney

Ah philosophy! Isn't it grand? 

For me logic is about making the correct  connections between things. For instance, if one believes in the Christian God, it is logic says that he/she doesn't believe in any other God.

Logic can lead you to the wrong conclusion if you make a wrong assumption. Since we do not know all the facts and what we do consider as facts changes quite often, stating that logic  leads you to the truth can be a misleading perception (I got to that conclusion by following "logic").

If I "know" for a fact that electric current cannot cause me any harm I can logically conclude that I can grab a live wire. Of course I will be electrocuted  but my actions will be based in a logical conclusion based on false facts.

The human society has some axioms and dogmas  which are entwined to our existence. How they have come about can usually be found out by using logic. For instance: "Murder is wrong". Well, we found out that it is safer to live in an organised society and that not having to fend for ourselves against our fellow human beings saved time which could be used in the society's advantage (I am bending the rules of logic here by jumping over several steps etc  ). We therefore came to the conclusion that it is better not to allow murders. We therefore created the axiom "murder is wrong". It is now part of our ethics and we don't question it. We take it as a given and I should stop now before I launch into a diatribe by a layman.


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## maxiogee

BlueWolf said:


> It isn't so simple however. Even simple readily observable events aren't sure. All these affirmations are now supposed as wrong:
> -Heavy objects fall more rapidly than light ones.
> -Time is something that is the same everywhere.
> -The Sun runs through the sky around the Earth.
> That does seem strange, since they don't "lend themself to substantial dispute".



They're not 'supposed as wrong' - they're just not held to be universally true, which is different.
Heavy objects do fall more rapidly than light ones - in certain circumstances.
Time is the same everywhere (where humans have so far managed to get to.)
The sun and the earth are not stationary - neither with regard to each other nor with regard to other parts of the cosmos, so the sun does move relative to the earth, even allowing for the movement of the earth.


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## BlueWolf

maxiogee said:


> They're not 'supposed as wrong' - they're just not held to be universally true, which is different.


My examples were all from science, and in science if you say that something is universally true and it isn't, your affirmation is wrong, regardless the fact it's true in some situations.


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## Abu Bishr

Hi Guys

This is my understanding of Logic. It is the system of rules that governs valid thinking or reasoning, or it is an instrument that guards against fallacious or flawed argument. It has been compared to a grammar of a language in the sense that just as a knowledge of the grammar rules of a language (or competence in the Chomskyan sense) enables us to distinguish between grammatical and ungrammatical sentences, so also logic enables us to distinguish between sound and unsound arguments (involving logical errors & fallacies). A typical argument involves premises and conclusions which are validly drawn from these premises.

Accordingly, Logic is not necessarily antithetical to religion, and to merely call religion "irrational" is to risk falling into logical fallacy. Afterall, many of the issues that we discuss in religion are often dealt with in philosophy, of which logic is a part. One of the most important branches of philosophy (at least before Wittgenstein and the Linguistic Turn) is metaphysics which deals with issues of ontology, nature of Reality or Being, the existence of God or gods, and so on. In fact, many philosophers themselves were theists, and attempted to build religion on a rational foundation. A very important stage in the history of philosophy was that of scholasticism or the philosophy of the Schoolmen. All students of philosophy are familiar with the traditional arguments for the existence of God: the ontological argument, the cosmological argument, the teleological argument, etc. We are also familiar with figures such as: Marmonides, St. Augustine, St. Anselm, Thomas Aquinas, Baruch Spinoza, Averroes, Avicena, Kierkegaard, and countless others. Some of the greatest practioners of logic were themselves theists. Aristotle to whom is accredited the title of "the Teacher" and who first formulated and codified the rules of logic, was himself a theist, and advanced, what he thought, logical arguments for the existence and nature of God. We are all familiar with his concept of the "Prime Mover".

Logic itself has today come under attack from many quarters. Foremost amongst these is Feminist Theory which refers to "rationality" as male subjectivity. In fact, postmodernism and poststructuralism have themselves been accused of leading to irrationality.

Science itself has been shown to be value-laden. Philosophers of Science such as Thomas Kuhn have shown that the foundations on which science rests is by no means absolute, but that scientific communities think within certain paradigms and ever-so-often a paradigm shift occurs leading to an upheaval and replacement of one paradigm by another.

Finally, it is not religion that is the problem, but people whose logic and reasoning are clouded with emotion whether they be theists or atheists. So, let us not assume from the beginning that religion is irrational, and incapable of advancing rational and logical arguments for its positions, for this position itself runs the risk of being logically untenable as well as being biased against religion, which is itself antithetical to the objective pursuit of truth.


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## malonso2

Your heavy object arguement?  Whats that?  In science we are assuming all else equal (air resistance, volume, time and height of release) those two objects are hitting at the same time.  Don't try to bend science please.


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## Conrado Herrera

BlueWolf said:


> http://forum.wordreference.com/showpost.php?p=1786417&postcount=49
> 
> _Let's have a candid conversation based on logic, not on religion or some other irrational belief system._
> 
> Since this forum is full of so logical people, I'd like some of them explain me what is logical in their point of view.
> What is your Truth, so that you can define something as logical/right? I'd like to understand, because when I hear these things, I always see people that say nothing can be considered objectivly true and right, but they all follow the "logic". Introduce me it, please, because I really have some problems to understand what it is.


 
When anyone talks about logic, essentially He refers "to Exist or not exist" "true or false", "Zero or one", but What really exist? What is really true?, What is really one and non zero? Then "logic is the set of truths" but you can be against logic (nor to be) Who wrote the set of truths? "It is really the logic" of course, we are not "the logic", Who or what is the logic?

Regards
Conrado


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## Hutschi

Why does logic not include: may be?

I think a logic that only included true and false is too poor to describe the reality.
Logic is a way of thinking and concluding. 

There may be more than one logic systems and methods. 



Best regards
Bernd


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## ireney

Who says that logic doesn't include "maybe"? Sometimes "the answer is 'perhaps' and that's definite" as a professor of philosophy  used to tell us.


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## Conrado Herrera

ireney said:


> Who says that logic doesn't include "maybe"? Sometimes "the answer is 'perhaps' and that's definite" as a professor of philosophy used to tell us.


 
maybe = perhaps = uncertainty 
This is the midway of The logic and that is not definite

regards
Conrado


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## Hutschi

For example, modus.irrealis says this.



> For me, a logical argument is one where if the premises are true, then the conclusion must be true


 
This implies:

1. I must definitely know whether the premises are true - to exclude wrong conclusions.
2. I cannot have partly wrong ideas about the conclusions.
3. It does not include Time.

I'm wondering, why logic does nether include time and development, except in Zenons concept who denies time.


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## modus.irrealis

Hutschi said:


> This implies:
> 
> 1. I must definitely know whether the premises are true - to exclude wrong conclusions.


I agree it does imply this, but I don't see what's wrong with that.


> 2. I cannot have partly wrong ideas about the conclusions.


I see things this way: if it's a _logical_ argument and you know (or believe) the premises to be true, then you know (or must believe) the conclusion to be true. But there are other kinds of arguments, inductive arguments for example, which only give a certain probability of being true to their conclusions. (There are logics that deal with more than just true or false statements, multivalued logics I think they're called, but I don't know much about them and I'm not sure I've seen arguments that can be based on them alone, since they just seem to fall back to arguments based on probability.)


> 3. It does not include Time.
> 
> I'm wondering, why logic does nether include time and development, except in Zenons concept who denies time.


I'm not sure what you mean here, by not including time.


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## maxiogee

BlueWolf said:


> My examples were all from science, and in science if you say that something is universally true and it isn't, your affirmation is wrong, regardless the fact it's true in some situations.


Your examples were true in the circumstances (evidence available) which prevailed at the time they were made.
 -Heavy objects fall more rapidly than light ones - when under the influence of the earth's atmosphere.
When vacuums can be created or used (space) then they fall at the same rate. This doesn't make the statement heavy objects fall more rapidly than light ones wrong, it just changes the conditions under which it is true.

 -Time is something that is the same everywhere. - again, this was 'true' before we changed our understanding of the nature of time.

 -The Sun runs through the sky around the Earth. Again, it needed scientific advances before we discovered that we were looking at this wrongly.

Logic only produces valid answers when the 'arguments' used are true. What we understand to be true can change with advances in knowledge. Truth can change with time, and sometimes what we know to be true may only be true _under certain circumstances_ - but we may have never discovered that it is possible that the circumstances (which we may omit mentioning) are not universally applicable - which is why we omit mentioning them.


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## Hutschi

modus.irrealis said:


> I agree it does imply this, but I don't see what's wrong with that.
> I see things this way: if it's a _logical_ argument and you know (or believe) the premises to be true, then you know (or must believe) the conclusion to be true. But there are other kinds of arguments, inductive arguments for example, which only give a certain probability of being true to their conclusions. (There are logics that deal with more than just true or false statements, multivalued logics I think they're called, but I don't know much about them and I'm not sure I've seen arguments that can be based on them alone, since they just seem to fall back to arguments based on probability.)
> I'm not sure what you mean here, by not including time.


 
Hi, modus.irrealis, I wanted to give just an example to the question.
There is nothing wrong with your definition of logic, if we consider, that it is the one mostly used. 

I always had problems with the classical proposition of the excluded third. (Satz vom ausgeschlossenen Dritten). 

The classical logic is just one way. You can have sentences and you can derive new sentences, and all are true.

When I was in school, we learned an example which I never will forget.

a) If the moon is made of cheese, the sun is made of butter.

is true, because the moon is not made of cheese. 

This was pure logic, and you can simulate it with electronic devises.

---

But: It helps a lot, all you can calculate follows such rules. It is very mighty.

---

But there is more.

We have the noun "Frauenlogik" (woman's logic). It was almost discriminating.

But if you think about it, in ,many cases, the results of this kind of logic are better.

----

Time and logic: The classical logic does not consider time.

They can not handle properly: When it is raining, the street becomes wet. 

Is this true or false?

When it is raining, the street becomes dry? 

It has only vague ideas, what is a heap or what is a wood. 

It cannot handle properly small changes in quality. 

Much of it can be done in multi-value logic, some in fuzzy logic, and some in woolly logic (as Terry Pratchett named it, when I remember right.)

Best regards
Bernd

PS: I think, the most important ideas logic gave us are paradoxons.


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## maxiogee

Hutschi said:


> The classical logic is just one way. You can have sentences and you can derive new sentences, and all are true.
> 
> When I was in school, we learned an example which I never will forget.
> 
> a) If the moon is made of cheese, the sun is made of butter.
> 
> is true, because the moon is not made of cheese.
> 
> This was pure logic, and you can simulate it with electronic devises.


But - what is the point of such arguments?
If you wish to bet on horses then you must abide by the real results, and not say that Horse A may have been beaten, but it is a better horse than Horse B, and as I backed horse A I should win even though it came second.

True logic has uses, what is the use of your example?
We can all be logical about impossible things, but it is like dividing by zero - it mau give infinity in mathematics, but nobody in real life ever divided anything by zero.




> Time and logic: The classical logic does not consider time.
> 
> They can not handle properly: When it is raining, the street becomes wet.
> 
> Is this true or false?


First set out the arguments - is there a street? Is it dry? If it is already wet then your logical answer is immaterial - so, why are ytou asking the question?



> It cannot handle properly small changes in quality.


It can if the arguments are soundly devised. 

Will I appear to lose weight, when standing on a weighing scales, if I exhale? Why?


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## Hutschi

> True logic has uses, what is the use of your example?


 
The teacher gave us the example with the moon as example for formal logic. The essence is: logic does not consider the contents. It gives only rules, independend on contents.  This way, it is very useful.

---

Will I appear to lose weight, when standing on a weighing scales, if I exhale? Why? 
Interesting question. You cannot solve it using only formal logic, you need contents.

I think, it depends on where you are. 

It may depend on temperature, on pressure, and on how exactly the scale works. (buoyancy, Auftrieb)

If you measure it and you are staying deep in water, you will become heavier when breathing out ...


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## modus.irrealis

Hutschi said:


> I always had problems with the classical proposition of the excluded third. (Satz vom ausgeschlossenen Dritten).


I've flirted with doing away with it, but I'm not sure what's gained by doing so. (Also, it's more common to say "excluded middle" in English.)


> When I was in school, we learned an example which I never will forget.
> 
> a) If the moon is made of cheese, the sun is made of butter.
> 
> is true, because the moon is not made of cheese.


I've learned about this as well, and it did seem very strange at first, but in the end it's very useful to say such a statement is true. For example it let's you say that "if p, then q" is equivalent to "either not-p or q" which does seem to be true.



> Time and logic: The classical logic does not consider time.
> 
> They can not handle properly: When it is raining, the street becomes wet.
> 
> Is this true or false?
> 
> When it is raining, the street becomes dry?


I still don't completely understand you. From my point of you "when p, q" is not a logical relation (the way "if p, q" is), so such a statement is "atomic" from the point of view of logic and its truth-value needs to be ascertained by other means (I'd say empirical investigation in this case).



> It has only vague ideas, what is a heap or what is a wood.
> 
> It cannot handle properly small changes in quality.


This comment, which I don't disagree with, makes me think you and I agree on most things but are just using different words, since I'd just like to limit the word "logic" to certain aspects, while you seem to take a broader view. I don't think this is a substantial difference. With this example, I agree that logic here doesn't apply (in the sense, that it doesn't give a solution to the problem), and I'd say what's needed here is a proper theory of meaning, which for me is outside of logic (since examples like this show you can't give the meaning of a word through a logical definition).



> PS: I think, the most important ideas logic gave us are paradoxons.


They are important, especially when they force us to confront what our premises ultimately entail, but I don't know about _the_ most important.


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## Redisca

BlueWolf said:


> It isn't so simple however. Even simple readily observable events aren't sure. All these affirmations are now supposed as wrong:
> -Heavy objects fall more rapidly than light ones.
> -Time is something that is the same everywhere.
> -The Sun runs through the sky around the Earth.
> That does seem strange, since they don't "lend themself to substantial dispute".


Doesn't matter.  Those things are still described as "true" or "false" and are not a matter of opinion or consensus.


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## BlueWolf

malonso2 said:


> Your heavy object arguement?  Whats that?  In science we are assuming all else equal (air resistance, volume, time and height of release) those two objects are hitting at the same time.  Don't try to bend science please.



I said anything about air resistance or volume, did I? When once they thought it was true, they didn't think that it was only if the air resistance and volume were equal.



> Your examples were true in the circumstances (evidence available) which prevailed at the time they were made.
> -Heavy objects fall more rapidly than light ones - when under the influence of the earth's atmosphere.
> When vacuums can be created or used (space) then they fall at the same rate. This doesn't make the statement heavy objects fall more rapidly than light ones wrong, it just changes the conditions under which it is true.


You don't need vacuums to prove it. Galileo hadn't when he said it was false. Just try with objects with the same weight but different form, and you'll see it.




> -Time is something that is the same everywhere.





> - again, this was 'true' before we changed our understanding of the nature of time.


Our understanding of nature of time changed after this theory, not viceversa.




> -The Sun runs through the sky around the Earth.





> Again, it needed scientific advances before we discovered that we were looking at this wrongly.


What precisely? An philosopher from Ancient Greek (I don't remember his name) already understood it thousands year ago. What did he have? Or Copernico? We didn't discover it when we went into the space! 



> Logic only produces valid answers when the 'arguments' used are true. What we understand to be true can change with advances in knowledge. Truth can change with time, and sometimes what we know to be true may only be true _under certain circumstances_ - but we may have never discovered that it is possible that the circumstances (which we may omit mentioning) are not universally applicable - which is why we omit mentioning them.


True doesn't change with time.  What we think it is changes with time. If the truth exists, it's always the same, it doesn't change with your knowledge or believes. And if we don't know in which circumstances it's true, it means we know nothing about it yet. When our ancestors knew this "truths", they didn't know the circumstances, and they built all their conception of the universe on them. When they discovered these circumstances, all these theories suddenly fell. Circumstances aren't important, they are vital.


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## maxiogee

BlueWolf said:


> True doesn't change with time.  What we think it is changes with time.



Yes, but we never know that we don't know all the circumstances. Therefore 'true' has to be taken as meaning 'True under the _conditions _which now prevail' - be those conditions _physical _or _conceptual_.

You mention that the ancient Greek understood that the earth went arpudn the sun - but I think you're wrong. I think that ancient Greek 'thought' that the earth went round the sun - this is very different from knowing it.

-----------------------------------------|
|................................*T**T*......|
|................................*TB*......|
| .*C*.....................................|
 -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
  -----------------|.......................|
----------------------\..-----------------
.......................\
.......................*F

* Imagine a field with a tall hedge around it.
There is a cow (C) in it, and there is a tree (T) in it.
The farmer (F) has bad eyesight and, standing at the gate his sees something brown beneath the tree.
He goes into his house and tells his wife "I know the cow is in the field".
Then his wife goes out and finds that there is a big cardboard box (B) beneath the tree, blown there in a storm. 
She brings in the box to the farmer and tells him it was what he saw beneath the tree.
Did he "know" the cow was in the field?


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## Conrado Herrera

maxiogee said:


> My dear dear Ireland man:
> don't get confused ourselves. The truth is true. Anyone can describe this situacion:
> The farmer do not know where is the cow but he guessed. The cow was there.
> These circumstances are not against or modify the truth or logic. Logic is absolute and have no fight against circunstances. As I see, the problem is inside you.  Logic is as simple as this: "to be or not to be", "one or zero", "correct or incorrect" "exist or do not exist".
> Let me tell you this story: there was a man so powerfull that he can do anything, if something was really blue and he says it is black, upon this moment blue is black, if something is really hot and he says it is cold, upon this moment hot is cold. What can we say about him? We can say he is "the logic" If you can find such a man, you have found what logic is. The logic is not an option of our whims neither of our circumstances.
> Regards maxiogee
> Conrado


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## Conrado Herrera

Hutschi said:


> Why does logic not include: may be?
> 
> I think a logic that only included true and false is too poor to describe the reality.
> Logic is a way of thinking and concluding.
> 
> There may be more than one logic systems and methods.
> 
> Logic does not describe anything, logic says if it is or it isn't
> Regards
> Conrado
> 
> 
> 
> Best regards
> Bernd


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## maxiogee

Conrado Herrera said:


> Let me tell you this story: there was a man so powerfull


No, there wasn't.
This is the very heart of the problem, Conrado,  your logic is concerned only with words - with thoughts.
Mine is concerned with 'reality'.

Regards.


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## ireney

Logic is just a tool!

If you have wrong "data" you will end up with the wrong result.
If you have insufficient "data" you will not end up with a conclusive result.

The tool is also useful sometimes to exclude something. With the process of elimination (reductio ad absudrum) for instance that's exactly what you do and it IS useful. Or to show that the conclusion someone else has reached are wrong (you take something that you consider/know to be false and consider it right; you proceed then to show that even if that was the case the conclusions drawn are still wrong). I believe that the only times when someone is willfully using wrong or hypothetical "data" when he deals with logic (that and explaining how logic works perhaps).

Ancient Greeks by the way most definitely didn't know that the Earth .
was round. That's why the  majority of them believed it was flat 

Oh and can someone explain to me the cheese moon example? I think I've missed something there


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## maxiogee

ireney said:


> Oh and can someone explain to me the cheese moon example? I think I've missed something there



The best I understand it, ireney, is If X, then Y.
But, if X is false then whatever falsehood you say about Y will be 'true' - for the purpose of the argument taking place.

If the moon is made of blue cheese, then the sun is
- a photograph
- the light on the front of an intergalactic express train
- the light inside God's refrigerator
 - - whatever you wish to say it is

That's my problem with those who go in for semantic logic - they don't use the 'tool' (as you so rightly call it) for any purpose, it is a toy to them with which they try to appear intelligent.


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## Hutschi

The good thing with this kind of logic is: it is formal and it gives you results. It has rules, and they are strongly defined and have no exceptions. If you use the rules, you will get results. 
And they are the base of all digital computers. 

The only problem: if you do not give any semantic to them, they do say nothing.

Once there was a time, when mathematicians thought, that all may be reduced to logic, and at least theoretically you could build a device that can decide if something is right or wrong. Than this concept proofed wrong. B. Russell tried it, but did not get it done. He found a paradoxon and could not solve it. Later Goedel found: If you have a non-trivial system, there are always propositions, you cannot decide in this system.

---

I think, there are several concepts of logic. 

Examples:

classical logic
propositional logic
multi-valued logic
fuzzy-logic
predicate logic 
first-order logic 
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intuitional logic ?
logic of plain good sense ?

---

logic of war ?


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## ireney

Oh! Thank you maxiogee. I think I sort of got it. 
But that is not even logic! 
If I say that If I am the Qeeen of Sheba then you are all Pink elephants with green stripes where's the logic in that? And let's say that some kind men put me in a padded room and prove to the world that I am not Her late Majesty. That still doesn't logically prove that you are not garishly coloured elephants.

If I said that "then and only then" you are stately (when unlucky) mammals then my not being a royalty would lead to the logical conclusion that you don't have tusks.

Even if we use logic as a play-thing we should at least use it right.

As for using it "practically". There's a famous (around here at least) little story about how two famous (notorious for others) sophists (?) twisted the words around using logic.  It always made me want to screem because, although it is fun to read, it could never, ever happen.


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## BlueWolf

maxiogee said:


> You mention that the ancient Greek understood that the earth went arpudn the sun - but I think you're wrong. I think that ancient Greek 'thought' that the earth went round the sun - this is very different from knowing it.



I see your point but once you've seen, heard, smelt a thing, do you think you know it? Following your reasoning, do we know the earth goes round the sun? Have you never been in the space and seen it? I don't know about you, but I haven't.


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## maxiogee

BlueWolf said:


> I see your point but once you've seen, heard, smelt a thing, do you think you know it? Following your reasoning, do we know the earth goes round the sun? Have you never been in the space and seen it? I don't know about you, but I haven't.



My point about the ancient Greek was that he (most likely) had no form of 'proof' - nothing to back up his assertion.

This is why I say that our 'truth' is not constant. 
We think we know something. We formulate a theory to explain it. The theory stands until it is either proved totally wrong, or (increasingly) proved to be not true in every detail - it needs fine-tuning.

That ancient Greek had neighbours who 'knew' there were numerous Gods who decided the fate of humans. His descendants have neighbours who have, generally, whittled this down to knowing there is one God, and that our fate is not 'decided' for us.


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## Conrado Herrera

Abu Bishr said:


> Hi Guys
> 
> This is my understanding of Logic. It is the system of rules that governs valid (consider like true) thinking or reasoning, or it is an instrument that guards against fallacious or flawed argument.
> 
> Thanks for this good explanation. As Ireney says, the logic is a tool. But when the feelings and whims enter, there is no logic.
> Regards
> Conrado


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