Category Archives: Why learn logic?

Sayers’ Helpful Summary of Logic

Sayers’ Vision for Logic

In her seminal essay “The Lost Tools of Learning,” the author Dorothy Sayers describes her understanding of the medieval scheme of education, specifically the Trivium — the three liberal arts of grammar, logic, and rhetoric. She argues that students in the Middle Ages were taught the proper use of the tools of learning by means of these arts. Of logic she says,

dorothy[1]“Second, he learned how to use language; how to define his terms and make accurate statements; how to construct an argument and how to detect fallacies in argument.”

As I have taught logic in the classroom, written logic texts (and blog posts), and spoken on logic and classical education around the world, I have regularly returned to this quote. It is for me perhaps the most useful sentence (of the 238 sentences) in the essay.

A Proper Pedagogical Progression

In this sentence Sayers explains what logic is for: logic teaches us how to use language. This reminds us that the liberal arts of the Trivium are language arts (whereas the Quadrivium are mathematical arts). Specifically, logic teaches us how to use the language of reasoning, of disputation and proof.

This sentence also describes a proper pedagogical progression of logic:

  1. We must start with terms: how to define them, relate them, and work with them, including understanding the value of defining terms.
  2. Terms are related in statements (categorical statements connect subject terms with the predicate terms). Logic teaches us “how to make accurate statements”; that is, how to make statements that are true and applicable, as well as understanding how we know that they are true, and how they relate to each other. It teaches how to do this with many different types of statements: simple and compound, categorical and hypothetical, immediate inferences, and so on. Terms are the building blocks of statements.
  3. Statements are the building blocks of arguments, as we connect premises together to draw conclusions. So logic teaches us “how to construct an argument”; that is, how to write a valid argument to establish a desired conclusion.  It teaches how to do this with many types of arguments: categorical and propositional, conditional and disjunctive, symbolic arguments and arguments in normal English.
  4. Finally, logic teaches us “how to detect fallacies in argument,” both the formal fallacies from the rules of validity for categorical syllogisms and propositional arguments, and the informal fallacies of ordinary discourse, like circular reasoning and ad hominem. Logic teaches us not only to detect them, but to name them, and to expose them by means of counterexamples to those untrained in logic.

Were I to add one element to Sayers’ list, it would be “to construct a proof in a step-by-step, justified manner.” With this addition, every page, every concept of both Introductory and Intermediate Logic is covered in Sayers’ helpful description of what is encompassed in learning logic.

Logic: A Science and Art

Is logic a science or an art? Of course, a logician would answer Yes, and here is why.

A science is a systematic study of some aspect of the natural world that seeks to discover laws (regularities, principles) by which God governs His creation. Whereas botany studies plants, astronomy studies the sky, and anatomy studies the body, logic studies the mind as it reasons, as it draws conclusions from other information. Logic as a science seeks to discover rules that distinguish good reasoning from poor reasoning, rules that are then simplified and systematized. These would include the rules for validity, of inference and replacement, and so on.

For example, logic as a science could study the apostle Paul’s reasoning in 1 Cor. 15, “If there is no resurrection of the dead, then Christ has not been raised… But Christ has been raised, and is therefore the first fruits from among the dead.” It then simplifies this into a standard pattern: If not R then not C, C, therefore R. This rule can be further simplified, named, and organized in relation to other rules of logic.

An art is a creative application of the principles of nature for the production of works of beauty, skill, and practical use. The visual arts apply their principles to the production of paintings, sculptures, and pottery. The literary arts produce poems and stories. The performing arts produce operas, plays, and ballets.

Logic is one of the seven liberal arts, which include the Trivium of grammar, logic, and rhetoric. These arts are the skills which are essential for a free person (liberalis, “worthy of a free person”) to take an active part in daily life, for the benefit of others. Specifically, logic as an art seeks to apply the principles of reasoning to analyze and create arguments, proofs, and other chains of reasoning.

In summary:

Logic is the science and art of reasoning well. Logic as a science seeks to discover rules of reasoning; logic as an art seeks to apply those rules to rational discourse.

Christian Logic

I was recently asked the question, is there a distinctly Christian view of logic? I offer here the beginning of an answer to that question. (I am not trying to be original here. These thoughts are from many sources. Just trying to be faithful.)

Laws of Logic

The laws of logic are universal (applicable everywhere), abstract (immaterial, grasped by thought), invariant (not changing), and authoritative (they must be accepted). A non-Christian worldview has a difficult time accounting for such laws. The laws of logic cannot be denied with any kind of consistency, since a denial of logic is tantamount to a denial of truth and reason. But if it is affirmed that the laws of logic are universal, abstract, invariant, and authoritative, yet not “from God,” how can they be justified? Where do such laws come from? They are not invented by men, because they would not then be universal, invariant, or authoritative. They are not material, because they would not then be abstract.

Rather, logic is an expression of God’s unchanging, orderly, truthful, authoritative character.

The Character of God

God Himself is logical; He is a reasoning being: “Come, let us reason together” (Isa. 1:18).  As the ultimate lawgiver He orders His cosmos in a logical way. “God is not a God of disorder” (1 Cor. 14:33). God is orderly, and order implies reason. Where there is no reason, there is only chaos. God’s word is truth (Jn. 17:17), and He would have us be truth tellers (Eph. 4:15). God Himself is non-contradictory. He is truthful (Jn. 3:33), He cannot lie (Heb. 6:18). He does not deny Himself (2 Tim. 2:13). John Frame, in his book The Doctrine of the Knowledge of God, identifies these attributes of God, and then adds: “Does God, then, observe the law of noncontradiction? Not in the sense that the law is somehow higher than God Himself. Rather, God is Himself noncontradictory and is therefore Himself the criterion of logical consistency and implication. Logic is an attribute of God, as are justice, mercy, wisdom, and knowledge.”

The Christian worldview does account for the properties of logical laws. The laws are universal because God is omnipresent; His character is expressed throughout His creation. The laws are abstract, needing no created, material foundation, because they existed before the creation, being attributes within God. The laws are invariant, because God does not change, and neither do His attributes. If the laws of what is true and rational could change, then how could God be trustworthy? How could He keep His covenant promises if truth could be non-truth? He can and does keep His promises, because Christ, the logos, is the same yesterday, today, and forever. The laws are authoritative, because God is the ultimate authority.

A Tool and Gift

God has communicated logic to man as a tool by which we can come to truth. God made us in His image with the ability to reason. We are created as rational beings, and God uses our reasoning ability to speak to us. For example, the giving of law presupposes an ability to reason. Laws are given in the form of universal propositions. “God has commanded all men everywhere to repent.” To obey this, we finish the syllogism: I am a man, therefore I must repent. Without logic, the command could not be applied to particulars. A denial of logic opens the door for disobedience, for without it we cannot obey.

Logic is presupposed, not only in law, but in all revelations of God to men. God gives us minds that reason just as He has given us eyes that see, in order that we may receive His revelation to us. Cornelius Van Til said, “The gift of logical reason was given by God to man in order that he might order the revelation of God for himself.” In order to comprehend any doctrine, we must use logic. The truth that there is one God, eternally existent in three Persons, though clearly contained in the Bible, is not found in any one place in scripture. To see the truth of the Trinity requires a godly, submissive use of logic. If a truth is truly and logically derived from the scripture, we have a divine obligation to believe whatever it is. This is what the Westminster Confession is referring to where it says, “The whole counsel of God concerning all things necessary for His own glory, man’s salvation, faith and life, is either expressly set down in scripture or by good and necessary consequence may be deduced from scripture.” Isaac Watts, the great hymn writer and logician, said it this way in his book on logic: “It was a saying among the ancients, Veritas in puteo, Truth lies in a well; and, to carry on this metaphor, we may very justly say, that logic does, as it were, supply us with steps whereby we may go down to reach the water…. The power of reasoning was given us by our Maker, for this very end, to pursue truth.”

Logic is thus a tool which God has given us in order to understand and obey Him. Like other tools, our grasp of it as humans is no doubt incomplete and imperfect, but it is sufficient for the task for which it is given. And like any other tool, we need to be careful how we use it.

Constructing Illustrative Parallels

In my last post, I claimed that there are three typical ways we use analogies: basic comparisons, ordered-pairs, and illustrative parallels. In this post I will explain how to construct an illustrative parallel, which is a powerful means of proof.

The Pattern

An illustrative parallel reasons from a particular example (the source) to a particular conclusion (the target). The process combines inductive reasoning (from the particular example to a general statement) and deductive reasoning (from the general statement to the particular conclusion) as shown:

I am fascinated by the inductive-deductive process that the mind goes through when reasoning by analogy, such as in the parables. For example, Jesus teaches in Matthew 5:14-15, “You are the light of the world. A city that is set on a hill cannot be hidden. Nor do they light a lamp and put it under a basket, but on a lampstand, and it gives light to all who are in the house.” The source (“no one lights a lamp to put it under a basket, but to give light to the house”) inductively implies the general intermediate conclusion that what is meant to illuminate something should not be covered, and that it is uncovered not in order to display itself, but something else. So when he deductively makes the particular conclusion in verse 16, “Let your light so shine before men, that they may see your good works and glorify your Father in heaven,” we understand that we should do good works, not to shine a light on ourselves, but that men might glorify God.


Inventing good analogies can be difficult, but we can be helped using the pattern above. Say that you want to use an analogy to respond to this challenge: “Why study formal logic? Everyone can already reason!” You could argue that the study of formal logic helps to improve our reasoning skills by providing standards to distinguish between good and bad reasoning. This is your target. It can be deduced from the general statement that studying a language art can provide standards by which we distinguish between the proper and improper use of that art. Given this, we must then invent a source, a different example of the general statement, and one that is preferably more familiar that the target. What familiar language art provides us with such standards? English is a good example; the study of English helps us improve our speaking and writing skills by providing standards to distinguish proper English from improper. The basic analogy could then be simply stated: “‘Why study formal logic? Everyone can reason.’ That’s like arguing, ‘Why study formal English? Everyone can speak!’”

Imitating the Masters

Jesus is, of course, the Master of analogies, as of all other forms of argument. But there are also many lesser masters from whom we can learn this art. My favorites include C. S. Lewis, G. K. Chesterton, Mark Twain, and Doug Wilson. Here are some of my favorites:

“I believe in Christianity as I believe that the Sun has risen; not only because I see it but because by it I see everything else.” ― C. S. Lewis

“The object of opening the mind, as of opening the mouth, is to shut it again on something solid.” ― G.K. Chesterton

“Laws are sand, customs are rock. Laws can be evaded and punishment escaped, but an openly transgressed custom brings sure punishment.” ― Mark Twain

“We have no structure any more. We have no shared creed. We do not know what we are here for. It makes no sense to speak of our inherited ‘shared values,’ or better yet, ‘core values.’ If they are arbitrary, shared values are worthless. If they are arbitrary, core values are simply located where our intestines are, and are full of the same thing.” ― Doug Wilson

“If we had no winter, the spring would not be so pleasant: if we did not sometimes taste of adversity, prosperity would not be so welcome.” ― Anne Bradstreet

What are some of your favorite analogies? Leave a comment!

Reasoning by Analogy

I have been thinking about analogies lately, and finding them fascinating. There appear to be three basic uses for the term analogy.


First, almost any comparison, especially one in which a familiar, simpler, or concrete thing is used to clarify or illuminate something that is unfamiliar, complex, or abstract, can be called an analogy. For example, this excerpt from George Orwell’s essay “A Hanging” is considered an analogy:

They crowded very close about him, with their hands always on him in a careful, caressing grip, as though all the while feeling him to make sure he was there. It was like men handling a fish which is still alive and may jump back into the water.

The manner in which the guards handled the prisoner is compared to men handling a fish. Most people have tried to handle a live fish just pulled from the water that wants back in, so this comparison gives the reader a vivid mental picture of the less familiar situation Orwell is describing.


Second, we see analogies in what can be called ordered-pair form: A is to B as C is to D, or more briefly A : B :: C : D. Typically these appear in the vocabulary or reasoning section of standardized tests, like this sample question from the GRE. Choose the analogous pair:

A. player : coach
B. child : parent
C. student : teacher
D. intern : doctor

The best answer is D. Just as an apprentice is training to be a plumber, so an intern is training to be a doctor. A child does not formally study to become a parent, and a player or student is not necessarily studying to become a coach or teacher (respectively).

Illustrative parallels

Third, we see analogies being used for the purposes of persuasion, called arguments by analogy, or what Aristotle calls illustrative parallels. Here is an example from Aristotle’s Rhetoric II.20:

Public officials ought not to be selected by lot. That is like using the lot to select athletes, instead of choosing those who are fit for the contest; or using the lot to select a steersman from among a ship’s crew, as if we ought to take the man on whom the lot falls, and not the man who knows most about it.

Illustrative parallels use both inductive and deductive reasoning. We use inductive reasoning to mentally move from the source (e.g. we ought not use the lot to select athletes) to a more general, unspoken intermediate conclusion (we ought not randomly select someone for a skilled position). We then use deductive reasoning to move from this intermediate conclusion to our specific conclusion, the target (we ought not select public officials by lot).

In my next post, I will explain how to construct illustrative parallels.

Logic with James B Nance

What will I learn in Intermediate Logic?

intermediate-logic-complete-program-dvd-course[1]Logic gives us standards and methods by which valid reasoning can be distinguished from invalid reasoning. It teaches students to think in a straight line, and to justify each step of their thought. Intermediate Logic does this using a symbolic language to represent the reasoning inherent in the language of argument. It is more flexible than syllogistic logic, and can thus apply to more real-life arguments.

Intermediate Logic Unit One teaches the powerful method of truth tables to determine the validity of propositional arguments. Unit Two takes these methods and teaches students how to deduce a conclusion from a set of premises, so they are able not only to show that an argument is valid, but also prove why it is valid. Unit Three teaches these same concepts using the modern method of truth trees. Unit Four applies these methods to the analysis of real-life arguments from 1 Corinthians 15, Hebrews 2, Boethius’ The Consolation of Philosophy,  Augustine’s City of God, and more (including a scene from the movie “Get Smart”). Unit Five teaches the fascinating application of these methods to the logic of digital electronics.

Gospel Enthymemes

Arguments in which one statement is left assumed are called enthymemes. Most logical arguments encountered in daily life are enthymemes. We can use the tools of logic to determine the assumption being made in an enthymeme.

Let’s examine three enthymemes in the Bible, all on the topic of Gospel salvation. Continue reading Gospel Enthymemes

Some Uses of Immediate Inference in Scripture

Logic students sometimes struggle with understanding and remembering immediate inferences. The more opportunities they have to see them used, the more likely they are to grasp them. Consequently, I want to give some examples of immediate inferences used in the Bible. Two equivalent immediate inferences for categorical statements are obverse and contrapositive. Continue reading Some Uses of Immediate Inference in Scripture

Why Standard Form?

Teaching students how to translate syllogisms into standard categorical form occupies several lessons in Introductory Logic. Lessons 11 and 12 explain how to translate categorical statements into standard form, which is then emphasized while learning about the Square of Opposition in Lessons 13-18. Lesson 19 teaches students how to distinguish premises and conclusions, in part so that in Lesson 20 they will understand how to identify the major and minor premises, so that they may know how to arrange a syllogism in standard order. Finally, Lessons 21 and 22 teach them how to identify the syllogism form using mood and figure. All this occurs before the students begin to learn how to determine the validity of a syllogism. Continue reading Why Standard Form?