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Would you classify logic as

  1. Oct 8, 2003 #1
    Would you classify logic as law or description? Are things actually GOVERNED by logic, or is logic simply a tool we use to understand these things?
  2. jcsd
  3. Oct 8, 2003 #2
    Thank God you didn't ask this when Alexander was here!

    Anyway, I have to go with "description", since there is no physical thing called "logic" and so "it" (logic) cannot exert physical control over anything.

    BTW, had you considered making this a poll?
  4. Oct 8, 2003 #3

    Tom Mattson

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    Logic is not a description, but a prescription.

    Logic pertains to the laws of reasoning. The description of how people do in fact reason is psychology, not logic. Logic is the laws that state how people ought to reason, and as such they are prescriptive in nature.

    It is the other way around: Logic is governed by "these things"!

    Logic is abstracted from everyday experience. We learn to make associations such as:

    "If I am cold, then I must find warmth."
    "I can either hunt or starve."
    "If I am cold and hungry, then I must find food and warmth."

    From countless such observations, we learn to extract the basic logical structure of those statements which are, respectively:

    "P OR Q"
    "(P AND Q)-->(R AND S)"

    That list is by no means exhaustive, but you get the idea. Furthermore, we learn more complicated logical forms by compounding experiences. Over the course of learning, we find that certain logical inferences are always valid, while others can be disproved by counterexample.

    For instance,

    "If I eat, then I will not be hungry. I am not hungry. Therefore, I will not eat."

    From observations such as these, we learn to abstract the following:

  5. Oct 8, 2003 #4


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    Actually, the Q predicate here is "I am not/will not be hungry." So what you have presented semantically is


    From this, it is not possible to derive the truth value of P. (If Q is true, then P-->Q is true regardless of whether P is true of false.)

    For the set of logical steps you presented, the derivation would be
    "If I eat, then I will not be hungry. I am hungry. Therefore, I have not eaten."

    edit: to make the above correspond more closely to our usual logic of the situation, we could say "I will not be hungry if and only if I eat. I am not hungry. Therefore, I have eaten."

    Symbolically this is

    From this we could also derive the above result: "I am hungry, therefore I have not eaten."

    Last edited: Oct 8, 2003
  6. Oct 8, 2003 #5

    Tom Mattson

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    Whoops! I typed too fast. I meant to illustrate modus tollens and botched up the verbal part.

    Change that to,

    "If I eat, then I will not be hungry. I am hungry. Therefore, I have not eaten."
  7. Oct 9, 2003 #6
    I'm not sure everyone understood this. Perhaps I can make the most sense through an analogy...

    Does math describe the world, or is the world GOVERNED by math? If I drop an apple from a certain distance above the earth, does it take so-and-so long to fall BECAUSE math dictates that, or does the math dictate that BECAUSE thats how long it will take to fall?

    That is my question, except with logic rather than math. If all possibilities but one have been logicly shown to be impossible, is the remaining possibility true BECAUSE of logic, or does logic dictate it to be true BECAUSE it is?
  8. Oct 10, 2003 #7
    The way I see it, the world might be governed by math but we derived it to describe our world, get what I mean?
  9. Oct 10, 2003 #8
    Sikz, the Universe as a whole doesn't understand mathematics (no sarcasm intended) or logic. The fact that something is logically necessary doesn't mean anything to the Universe as a whole (since the Universe is not conscious). Thus, your other alternative must be true (even if just by default) - we must have come up with logic to describe (and prescribe) the way things "ought to be" based on patterns that we observed in the past.

    Perhaps this old thread of mine would be relevant: A Universe Without Logic.
  10. Oct 10, 2003 #9


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    "Logic is a wreath of pretty flowers, that smell bad!" - Mr. Spock
  11. Oct 10, 2003 #10
    Spock actually said that?!
  12. Oct 10, 2003 #11


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    Yes. The ship was taken over by androids, and they discovered that displays of faulty logic hampered the android's thinking processes. They defeated the android leader when Kirk told him, "Everything I say is a lie."

  13. Oct 10, 2003 #12

    What inane balderdash! I can't imagine this having any adverse effect on a Vulcan person, so why should it effect a logical android?
  14. Oct 15, 2003 #13
    I would also classify logic as the authority I try to listen to above all other authorities. Things aren't govened by logic because it's just a word that is a very old refinement into more verbal or visually symbolic ways of the natural ability of the mind to put 2 and 2 together that sometimes comes out to 5 but the more one uses logic the more things seem to be.
    I use to pretend I was Spock, he was the one of the great nerd leaders.
    Last edited by a moderator: Oct 15, 2003
  15. Oct 18, 2003 #14
    Tom and Hypnagogue,

    Just from the little I've read here I think I like logic.

  16. Oct 21, 2003 #15
    That is probably the logical choice.
  17. Oct 28, 2003 #16
    Not sure why Tom calls logic as laws of reasoning, giving it some gist of absoluteness. I thought logic is more like a set of rules that help us to evaluate validity of reasoning.
    I mean, these rules aren't necessarily unique and absolute. Their main strength is that they are internally consistent, and thus usable to evaluate internal consistency of reasoning statements. That internal consistency doesn't give the rules any substance nor universal uniqueness.

    I also don't understand why its so often said that Logic is extracted from everyday experience and is thus governed by it. I find the only correlation between logic and everyday experience in that both are "consistent". That doesn't make either more important than other. Infact it seems that "consistency" is actually the only true property of any logical system. To say that everyday experience is source for our Logic system is to say very little, much like saying that our existence is source of logic. Our logical ruleset is very strongly reduced set to keep it manageable and reasonably simple, ie. efficient. Whether its the only one or even the best one isn't very clear. Thus Logic system isn't something unique or absolute, but more like a "theory". Being manmade, its no worse nor better than any successful theory, subject to changes.

    Imo, logic is "tool", to understand when we are talking gibberish, no more. Logic can't ever produce anything, only help to validate our constructs. It can't even tell if something is true, only that its valid as statement. To attribute to logic some world governing powers seems excessive.
  18. Dec 10, 2003 #17
    On logic

    Logic is neither of the 2 categories you just gave.

    Logic is more of a method. It has no real laws in the same way you have scientific laws. The general guidlines of logic schemes, or paradigms, could technically be construed as laws that ought to guide, and correct the mind, but logic is conditional in the sense that its effectiveness is totally related to the amount of knowledge one has.

    To be logical, in the truest sense only means that one has exhausted their thinking skills to the maximum degree so as to achieve some kind of resolution to an equation.

    There was once a mental game exposed to me once, when i was younger, that asked me the question of how i would get 3 items accross a bridge. These were the conditions i had to work with:

    a]. I have come to a bridge with 3 items.
    b]. I have a fox, some hay, and a sheep.
    c]. I can not carry more than a single item accross the bridge.
    d]. The sheep eat the hay, so you have to be careful about leaving the sheep behind, and taking the fox ahead first instead.
    e]. You if you choose to take the hay first, and leave the fox with the sheep first, then you run the risk of having the sheep be eaten alive by the fox.

    The above was an excercise of logic.

    In relation then to the above, to use logic, or to be logical, would be to figure out how it is that you are going to cross the bridge with all of the above rules not being violated.

    In a game of chess, to be logical, would be to make the most effective moves possible. Hence, by default, to be illogical would be to make stupid moves, like putting the Queen in a spot where she will be eaten by another piece, especially low piece like a pawn.

    In this sense then, logic is simply the capacity to methodically manipulate a bunch of variables so as to resolve an equation. It is applied, and disciplined thinking that runs through every conceivable factor/variable so that you know how to handle, or survive a particular situation.

    But as stated, logic is dependant on knowledge, for your knowledge base will influence the logicalness of certain conclusions.

    Wisdom, is the essence of logic, for you can not do better than wisdom.
    Last edited by a moderator: Dec 10, 2003
  19. Dec 10, 2003 #18
    Are you sure yur being logical?

    You seem to missunderstand the very idea you want to cast judgements onto.

    Reasoning, is synonomous with logical thinking. To reason, is to apply logical thinking to a task.

    They can be laws in the sence, that they furnish the mind with guiding principles. It is through these principles, that assists the mind to understand whether or not a certain train of thought is logical, or irrational. In this sense then, logic is more than a set of rules. It is a method of thinking that is different from just wondering off, without any sense of direction.
    As David Hume had long ago proposed, the mind has impressions that result from the senses. Hence, to have thought, requires that there is already an unknown number of inprints made upon your mind so that you may contemplate upon them. You can not contemplate upon the meaning of the sunset, if the very idea of the sunset does not already exist in your mind.

    Because, the mind must do something with all of its experiences, the mind furnishes what are called generalities. These generalities must either be logical, or illogical. What gives something logic, is the fact that you can use an actual experience from your memory bank as a case in point for why a particular axiom is logical to hold as an axiom, and why another axiom must be abandoned since it can not be supported by any retrievable experience that relates to the subject matter that you want to analyse. In this since, yes, everyday experiences can provide you with general observations about some aspect of life. Once you have this generalisations, you can then deduce logical predictions, such as, "i will more than likely not survive a bullet to the skull if i put the gun to my temple, and pull the trigger." How can i be sure this is a logical assumption to make, as opposed to an illogical one? Simple: bullets kill, and bullets have been known to penetrate many objects with no problem. Hence, i can logically deduce some kind of opinion from what i know of things in general. My conclusions/opinions then must have some deagree of rational thought behind them, otherwise why would we submit to various notions at all?

    I also known from experience, that more often than not, people respond to kindness, than to hostility alot better, hence, it is only logical that i tailor my behavior so that i may be in harmony with as many people as possible during certain situations. Sounds pretty logical to me.

    It's a tool in the sense that you can use your logic for some task. It may not be a tool like a wrench, but it is a tool, in the same way your legs can carry you from spot A, to spot B. The mind is a tool, its how you can calculate, and solve some problems.
    Last edited by a moderator: Dec 10, 2003
  20. Dec 10, 2003 #19

    Tom Mattson

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    It's been a while since I looked at this thread...

    What is the difference between what I call a "law" and what you call a "rule"?

    Right. Scientific laws are descriptive, and cannot be broken. The laws (or rules or method, if you prefer) of logic is prescriptive and can be broken.

    Right again. Not only does one need a knowledge of the methods of logic per se, but one also needs knowledge of the subject about which one is reasoning. Logic itself is quite sterile, consisting of relationships between logical variables through unary and binary operators. Even if someone has trained their mind to make only valid inferences, it doesn't matter at all if that person starts out with one or more false premises.

    The GIGO rule is very much in effect: Garbage In-->Garbage Out.
  21. Dec 10, 2003 #20
    Hello, i am Mr Insanity, how are you?

    Well, technically speaking, i suppose theres no real difference other than semantics, politics, and personal preferences.

    To me a rule doesnt necessarily have to have an actual bases. I can for example make it the rule to never walk into the house without my shoes. A law on the other hand seems to imply a rule that is not really of your own making, but rather that is a part of, or the result of something that defines a particular observation.

    For example, i can say that that all life requires some kind of energy (as a law) in order for it to be alive, and thus an example of life, as opposed to simply having existance. Just because something exists, doesnt mean its alive, so if its not alive, it not be considered a life form, since life forms have something in addition to having mere existance. A stone has existance, but it isnt alive, since it has no life. Hence a law of life is to have life forms that must feed off of something else in order for it to not only be a living thing, but to have survival.

    In this manner, a law is a rule that is not really of my own making, and that it seems to be pretty much a rather consistant thing when it comes to some kind of observation. But over, and beyond that, theres really no difference.

    Another way to understand the power of a truly logical law of thought would be something like math: "2+2=4". This is not just a made up rule, but a fact that must be universal when it comes to adding quantities. In this sense, it is logical to accept the premise that "2+2=4".

    I am not sure i agree with any of the above.

    To break something has more than one meaning. In once sense, you can say you have violated a rule, and in other since, you can drop a glass, and watch it break into pieces. These 2 cases are both examples of brackages of some kind.

    Anything dealing with words is descriptive, hence knowledge in a certain since is a description of something that relates to a thought of some kind.

    No, logic is not prescriptive in the sense you think it is. Logic has a valid authority that is not arbitrary as a ramdon thought thrown on a piece of paper might be. I think people confuse a logical thought with an imperative in the sense that somehow you can not override a logical advise. But saying so, does not mean that there isn't such a thing as a wise action. Whether or not you choose to act wisely, has no bearing on whether or not there is such things as logical principles.

    Science is the result of logic. Hence, if Science is unbreakable, then so is logic, for without logic, you would not have science. All the laws that govern correct thinking, or wise thinking, must give way to meaningful fruite. Or as i have read in the New Testament somewhere, "A tree is known by the fruit it bears." Logic must have its consequences, in the same way foolishness must have its mark.

    I am not sure i know what you mean by the last sentence. What gives validity to a train of thought?

    At some point, true reasoning must rely on an unshakable foundation if there is to be power in the way one goes about and conquers a challenge. In a game of chess, all your reasonning must have some basis in reality, hence your validity stems from what is reality. If your whole system is based on a total absence of facts, then you can not make a valid inference, since you are only guessing, and not working from principles that have some kind of "integrity."
    Last edited by a moderator: Dec 10, 2003
  22. Dec 10, 2003 #21
    Re: It's been a while since I looked at this thread...

    Of course, english is such a nice language, each word having 15 meanings you can pick from, so maybe what you meant by "laws" was really same thing. I'm sorry then.

    The difference I saw is that "law" has gist of absoluteness, that doesn't tolerate competing options - it either is, universal, or it isn't, ala energy conservation law. Set of logic rules doesn't carry such absolute uniqueness, there are no strict limits to number of rulesets that can be applicable with same degree of success (fuzzy logic vs boolean logic comes to mind).

    Thats when I thought about property of "internally consistent". I suppose, that two sufficiently different logical systems might even look inconsistent to each other (when evaluating from other system), can be internally consistent by its own rules, and be both equally successful in dealing with real world, although perhaps very differently.

    Our way of thinking and our logic rules are really rooted in cause-and-effect experience. Only recently we've had hints that this might not be absolute state of affairs. And seems that to deal with that our logical system becomes increasingly inefficient.

    By braking, you really mean "ignoring" here I think. Both can be ignored. In one case you'd get killed, in other, well, just flamed.
  23. Dec 11, 2003 #22

    Tom Mattson

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    Re: Hello, i am Mr Insanity, how are you?

    You seem to have missed my emphasis on the difference between descriptive laws and prescriptive laws. The "rules of our own making" are prescriptive, while the "rules not of our making" are descriptive.

    I don't think there is anything universal about mathematics. The truth value of the proposition "2+2=4" depends on the definitions of "2", "+", "=" and "4".

    Obiously, when I speak of laws being broken, I mean the first sense of "breaking".

    Right, but some things "dealing with words" describe a prescriptive law. The US Constitution has words that describe legal and illegal conduct. Other things "dealing with words" describe a descriptive law. Physics textbooks describe (approximations to) the laws of nature, which themselves describe the workings of the universe.

    The fact that both are expressed in words (or in mathematical terms) does not in any way erase the distinction between the two different uses of the word "law".

    I did not say that logic is random or not authoritative. I said that we determine what valid logic is through experience, by finding out which inferences "work" and which do not.

    That's what I said.

    You are making a fallacy of equivocation here.

    In the first instance of the word science, you are referring to "the scientific method". That is indeed the product of logic. It is prescriptive. That is, it tells us how science should be done. But it is not unbreakable, because there is nothing stopping people from disregarding it.

    In the second instance of the word science, you are referring to "the laws of nature, as revealed by the scientific method". Those are not the product of logic, but of observation and induction. They are descriptive. That is, they tell us how nature actually behaves. And they are unbreakable: If you step off the Empire State Building, you will fall according to the law of gravity, whether you want to or not.

    Right, I agree.

    I mean logical validity. An inference is valid if, when used with true premises, it always yields a true conclusion.

    Logic has a strong foundation, but it is a myth to think that it is "unshakable". Logic itself cannot be justified by logic, no matter how many "meta" levels you want to explore. Logic is justified experientially, by induction. That is, inferences are accepted as valid because they work.
  24. Dec 11, 2003 #23

    Tom Mattson

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    Re: Re: It's been a while since I looked at this thread...

    Maybe we mean the same thing then.

    I think you also did not get my distinction between descriptive laws and prescriptive laws. Energy conservation is of the former type (descriptive), and logic is of the latter (prescriptive). The a posteriori laws of science and the a priori laws of logic are like apples and oranges.

    Right, first order and predicate logic is applicable when two-valued (T or F) logical variables are applicable. Certainly there are different logics that apply under different circumstances. That does not make any of them any less "prescriptive".

    I agree that our two-valued logic gets its justification from experience. I also agree that any one system of logic is not to be used under all circumstances.

    No, I don't mean "ignoring", I mean "violating". You can't violate the laws of physics, try as you might.
  25. Dec 12, 2003 #24
    Re: Re: Re: It's been a while since I looked at this thread...

    Well, I get (and did) the meaning of both, but I don't see why you stress this.

    Well, you can't really "violate" laws of logic, either.. By using logic, you are constrained by its rules. You can't violate logic while following its rules. You can only ignore logic.
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