What Are the Laws of Logic?


There are three basic laws that all people use when they engage in rational discourse. These are [1] the Law of Identity, [2] the Law of Non-Contradiction, and [3] the Law of Excluded Middle. Here we shall briefly describe these laws and why we are rational to hold to them.

The Law of Identity states that everything has an identity. Central to identity are certain characteristics that constitute the entity (a ball, person, dog, watch). An entity without an identity cannot exist because it would be nothing, therefore, for any entity to exist it must exist with an identity. According to this axiom, an entity cannot possess two identities, for example, a tennis ball cannot be a table, and a truck cannot be a dog. Moreover, an entity can possess more than one characteristic, such as a tree being both rough and brown.

The Law of Non-Contradiction says that a statement cannot be both true and false. It asserts that the two propositions “A is B” and “A is not B” are mutually exclusive. We all use this intuitively. If a scholar suspects that the conclusion to her deductive argument contradicts one of her premises then she knows she has to go back to evaluate the premises. Why? Because she realizes that she made an error in logic and that her conclusion cannot logically follow. Another example could be a mother suspicious of her son’s story of what he had been doing with his friends the day before. Why? Because when she asks him questions about it he recounts multiple events that appear to contradict each other. The mother thus suspects her son of getting up to mischief because of the imaginative lengths he goes to, to convince her by harmonizing contradictory accounts.

The Law of the Excluded Middle says that a statement such as “There is a cup on the table” is either true or false, and that there is no other alternative. In other words, the so-called “middle” position, that the cup “is both on the table and not on the table” is excluded on logical grounds.

Now, these are not arbitrary laws that philosophers simply invented nor are they arbitrary constructs. Rather, they have a long history in philosophical development stemming back to Plato, and they also appear to be unavoidable in the sense that they govern all thought and rational discourse. Without them, rational and constructive discussion, debate, and criticism would be impossible.

There are two primary reasons for affirming these laws. First, those who deny them undoubtedly use them in their denials. This not only demonstrates that the laws are unavoidable but that to deny them is self-contradictory. Second, these laws are intuitively obvious and self-evident. In fact, so much so that the burden of the proof would be on the skeptic to provide a logical defeater of them.

Further, what does it mean to call the above-mentioned laws “universal,” and what is meant by them being called “laws?” First, when philosophers refer to them as universal, this means that they are universally applicable. They do not only apply to some events or occasions. For example, that a traffic light is not a dog (the Law of Identity) is not only true in Africa but also in Asia. Second, they are laws in that they are unbending, without exception, and that deviation from them is impossible. And finally, these laws are discovered. They were not invented by people.



  1. Interesting piece. As these laws of logic obviously pre-date humanity, and must have existed for all time, may I ask who or what created them?

      • Speaking as just an ordinary bloke with reasonable education and intelligence I believe that the laws of logic emanated initially from a Supreme Being, along with all the other non-physical laws and concepts such as thermodynamics, gravity, mathematics, good and evil, right and wrong, objective morality, etc. To me there has to be an unimaginatively-high and supremely-intelligent authority which is also responsible for the creation of an orderly and finely-tuned universe in the first place, of course.
        I don’t think one has to be at all “religious” in order to believe in some great designer and architect behind everything – which many, including myself, choose to call God – and which just leaves me with a profound sense of awe and wonder. I have a great respect for science and all its discoveries, but it is those metaphysical areas which science cannot examine that interest me the most,
        No doubt someone of your high learning and intellectual ability can easily shoot me down in flames, but you did ask!

  2. The law of identity might have to be revised to take into account evolution over time whereby species continue to change over time.

    Even matter and energy continue to change over time and circumstances as suns and planets form, dissolve, reform, and as earthly molecules and minerals get reshaped over time from rocks to living organisms to computers.

  3. There is nothing inherently supernatural about the laws of logic.

    Logic is so basic it is self-referential and axiomatic, starting with “A=A” just as math begins with “1=1.” 

    The ability of humans to mimic each other and nature, as well as spot when two things are similar is basic to learning, since much of logic consists of making basic comparisons as to what resembles or does not resemble a thing or activity, or is equal or unequal, bigger/smaller, great-than/less-than, 0 or 1, etc. Such idiomatic recognitions do not appear to prove anything about “God.” 

    Also, the world of designated symbols like 1, 2, A, B, is not the world at large, a world filled with definitions that overlap, and a world with vagueness due to inherent limitations of words. 

    Librarians recognize the difficulty whenever someone writes a book that crosses genres or crosses scientific disciplines, or when geographical boarders and names of countries change, or word usages and meanings change over time. Psychology is like that too, attempting to define people’s behavior patterns using a limited number of ideas and terms, but people’s dispositions and motivations lie along a spectrum, making strict categorizations of each person’s “psychology,” difficult to say the least. 

    Anyone can form their own system of categorization, but all systems require perpetual tweaking, from the Library of Congress System — to Google’s system of ranking links via algorithms that seek hub sites that link to other hub sites with a similar focus, and comparing number of “hits” at each hub for different words — which also provides an analogy for how a neural network functions.

    Some categorization systems are more comprehensive and more capable of absorbing new categorizes as they arise or change, while others grown more unwieldy over time as categories continue to multiply and change.

    What about the vagueness inherent in words themselves, the fuzziness? Take the word “heap.” If you start with a tiny particle of something and keep adding more, then exactly at what point do all the particles become a heap? If you slowly expanded the width of a chair, at what exact point would it no longer be a chair but a couch? Or think of the many things upon which one might sit upright, called “chairs,” everything from the standard four legged chair to an amorphous bean bag. There is no divine “chair” in some world of Platonic absolutes. Speaking of fuzziness, if we had the technical ability to replace individual base pairs in the DNA of a chimpanzee, making each of its genes more closely resemble those in the DNA of a human being, after which replacement of which DNA base pair could you now declare the chimpanzee to be a human being? What if one reversed such an experiment, changing a human being into a chimpanzee one DNA base pair at a time?

  4. Regarding the laws of logic, you say that “human beings did not invent them” but you do not state who or what you think actually did.
    Also, would you agree that if these laws of logic apply throughout the cosmos, then they must have been created (along with all the other fundamental laws) at the same time as the Universe itself, and by supernatural means.

  5. I agree with your evaluation of the laws of logic but one of your examples was mistaken. As you stated, the Law of Non-Contradiction states something cannot be both true and false and the Law of the Excluded Middle is it must be either true or false. However, stating the Law of the Excluded Middle excludes the possibility that the cup is both on the table and not on the table is incorrect. The Law of Non-Contradiction does that. The Law of the Excluded Middle excludes the possibility that the cup is neither on the table nor not on the table. That is, the Law of the Excluded Middle states that at least one of the possibilities must be true. Taken together with the Law of Non-Contradiction, this implies that exactly one of the possibilities is true.

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