What Are the Three Laws of Logic?

Source: The Apologetics Study Bible (YouTube video)

by J. P. Moreland (website)

There are three fundamental laws of logic. Suppose P is any indicative sentence, say, “It is raining.”

The law of identity: P is P.

The law of noncontradiction: P is not non-P.

The law of the excluded middle: Either P or non-P

The law of identity says that if a statement such as “It is raining” is true, then the statement is true. More generally, it says that the statement P is the same thing as itself and is different from everything else. Applied to all reality, the law of identity says that everything is itself and not something else.

The law of noncontradiction says that a statement such as “It is raining” cannot be both true and false in the same sense. Of course it could be raining in Missouri and not raining in Arizona, but the principle says that it cannot be raining and not raining at the same time in the same place.

The law of the excluded middle says that a statement such as “It is raining” is either true or false. There is no other alternative.

These fundamental laws are true principles governing reality and thought and are assumed by Scripture. Some claim they are arbitrary Western constructions, but this is false. The basic laws of logic govern all reality and thought and are known to be true for at least two reasons: (1) They are intuitively obvious and self-evident. Once one understands a basic law of logic (see below), one can see that it is true. (2) Those who deny them use these principles in their denial, demonstrating that those laws are unavoidable and that it is self-refuting to deny them.

The basic laws of logic are neither arbitrary inventions of God nor principles that exist completely outside God’s being. Obviously, the laws of logic are not like the laws of nature. God may violate the latter (say, suspend gravity), but He cannot violate the former. Those laws are rooted in God’s own nature. Indeed, some scholars think the passage “In the beginning was the Word [logos]” (Jn 1:1) is accurately translated, “In the beginning was Logic (a divine, rational mind).” For example, even God cannot exist and not exist at the same time, and even God cannot validly believe that red is a color and red is not a color. When people say that God need not behave “logically,” they are using the term in a loose sense to mean “the sensible thing from my point of view.” Often God does not act in ways that people understand or judge to be what they would do in the circumstances. But God never behaves illogically in the proper sense. He does not violate in His being or thought the fundamental laws of logic.

Comments

  1. Samuel Heffner says

    Could you further explain the second reason a little better? “Those who deny them use these principles in their denial, demonstrating that those laws are unavoidable and that it is self-refuting to deny them.” Perhaps an example? I can’t quite get my head around how someone would use the principles in denying the 3 laws of logic. Thanks in advance,
    SBH

    • Admin says

      Sam, thanks for the question. If you want to make an argument, you need to use these laws in your argument. If you don’t the argument will not make sense.

      So, to argue against any of these three laws of logic means that you are using the laws to disprove the laws. Kinda like sawing off the branch you are sitting upon.

      Does that help?

      • Blake says

        Sam,
        For your example lets use the example, “it is raining” person A uses the law of identity to state that it is is raining but person B doesn’t agree and uses the same law to say it isn’t raining the law of non-contradiction then comes into play and says it can’t be both raining and not raining in the same place at the same time (assume for this example that both persons are in the same place at the same time) now law three is really where the discussion started because it either is raining or it is not raining. It can’t by the third law be both.

        These laws come into play in all discussions without even recognizing them. Knowing them is certainly better for anyone giving answers(apoligetics) because they will know when others are violating them and so correct the mistake gently. This is also why it is important for laying boundaries on discussions because if in the heat of the moment person B abandons all logic and turth they will go beyond logic and truth in order to prove their point. “Its not raining because I say so”,

        And just a biblical example to drive it home when the Pharisees tell people that Jesus casts out demons at the command of demons Jesus comes back with the law of non-contradiction saying a house divided cannot stand the devil casting out devils makes no sense then the law of identity Jesus being the messiah can cast out demons because he has the authority over them( so do we!(: ) and then he asks them about the third law basically I am either who I say I am which the evidence says he is or he is not there is no middle.

        I also hope these examples are sufficient.

  2. Laszlo Bencze says

    This is an extremely comprehensible explanation of these fundamental laws of logic. The response of many non philosophers to these laws is that they are so obvious that it’s silly to have to state and explain them. But of course it’s very necessary to have them clearly understood because it’s actually very easy to forget one or another of them and present absurd arguments.

    Case in point regarding the second law: evolutionists often argue that the extreme perfection of living things (the elegance of DNA replication, the acute eyesight of the eagle, the intricate musculature of the elephant’s trunk) prove evolution because such perfection must be the result of innumerable steps of fine tuning through the eons. Yet, at the same time they also argue that the imperfection of living things (junk DNA, the useless human appendix, the inverted retina of the human eye) proves evolution because such mistakes are bound to occur in a random process. So we have both perfection and imperfection offered simultaneously as proofs of evolution and that is simply contradiction.

    • Michael says

      (Forgive me for bringing up scientific discussions in a philosophical area.)

      I would like to commend you for your examples revealing the double standard, Laszlo, but for clarification’s sake I’m going to correct this for anyone else who come’s around: The real story is even juicier!

      There exists no definitive proof that anything is “vestigial” or “useless.”

      As more research is done, the more evidence to the opposite effect is found to be true. All of the examples of functions which are “useless” have been proven false. All of your examples of useless functions, and all of the other examples I’ve seen brought against Christian’s, have been thoroughly debunked to my knowledge.

      The majority of those claims are made when the scientist doesn’t know what it’s function is. Simply because a function is unknown doesn’t mean we should title it as “Useless.” This is scientifically dishonest when no real research has been committed to the subject at hand. Further study always shows that those “useless” portions of the genome, body parts, or so on, are all useful for the purposes of the organism.

      You mentioned junk DNA, the useless human appendix, and the inverted retina of the human eye. These are commonly used, but are false in their own ways.

      Junk DNA actually has several uses for the function of the living organism.

      The more research that is done on DNA, the more it proves that it has key uses. Everything has a function. As one scientist well said, “Those of you who still think junk DNA is junk, I invite you to take it out of your genome and see what happens.”

      Much in the same vein of things, vestigial organs which were thought to be useless have been found to have use. Removing them, much like with “junk DNA”, can bring effects which range from “disadvantaged” to “disastrous.” Some may promote the immune system, for instance, while other appendages or bones/organs are key for mating or carrying and birthing children during pregnancy. Others, like the human tailbone, are required for using the toilet and even just sitting.

      As for the inverted design of the human eye? Research on the vertebrate retina indicates that the existing inverted design in vertebrates is superior to the verted design.

      It is true that our eye would be poorly designed if we were sea-dwelling creatures at the bottom of the ocean, but we are most certainly not. The inverted eye is of superior design when compared to the verted eye of other creatures. Indeed, research has shown that our eye design allows us to capture the details and contrast in fully fleshed out hues of color and depth.

      I’ve gone through all three of your examples, and I’m certain more could be raised. That wasn’t an exhaustive list, and by no means was this one either. I know of a few more that I’ve seen proposed. But ultimately, they don’t hold water.

      The claim that there are useless or poorly designed functions is illegitimate in light of the overwhelming scientific evidence to the contrary.

      “I will praise You, for I am fearfully and wonderfully made. Marvelous are Your works, and that my soul knows very well.” – (Psalms 139:14)

  3. ep says

    Logic

    god created everything, therefore god created evil
    god created evil, therefore he is evil
    god is evil so he can not be good
    god cannot be good and evil at the same time

    • Admin says

      Hi Windchess, unfortunately there are gaping holes in your logic.

      1. Evil is not a thing, it is a lack of a thing, just like darkness in not a thing, it is the absence of light.

      2. For you to judge God as evil would require you to have a moral standard from which to judge. Where is this standard you are using? Can you show it to me please?

      What do you think?

      CA

    • Blake says

      Logic never supports whether someone is evil or good. Logic is simply reason and saying god is evil because he created evil is like saying your a sanwhich because you created a sandwhich…. that doesnt seem logical? God also created good god shouldnt be judged according to your ideasof good and evil because he created them just like that sandwhich isnt going to come to life and say you did a horrible job making it!

  4. David says

    Hi…Can you help me out please…Who is responsible for the law of identity ? i.e who came up with it ? Was it Aristotle ? I find conflicting information via google and i want to use the law of identity in a masters paper which of course I have to credit to someone, although ti is quite likely I cannot use it as an academic perr reviewed statement :)

    Appreciate any help

    • Admin says

      Hi David, there is a history of the LoI here:

      en.wikipedia.org/wiki/Law_of_identity

      I guess you are asking who formalised the LoI. Informally we use it all the time without thinking about it and presumably would have dome so since the beginning of humanity. Philosophically the interesting question may be whether that law could exist if God did not exist.

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