Why Study Logic?

by John W. Robbins  

If you are thinking of reading this book or taking a course in logic, then you need reasons for doing so. Why study logic? What can logic teach us that chemistry or history cannot? Can logic teach us anything, or are the mysteries of life deeper than logic? If you intend to study logic only because your course of study demands it, then another question immediately arises, Why does the curriculum demand a course in logic? Why did anyone think logic was important enough to make it a required course? There are questions that deserve an answer, but the answer may not be exactly what you might expect. Because many people disdain logic, it will be necessary to understand the relationship between logic and morality, for example. After all, many people think one should not study logic. "Life is deeper than logic," we’re told. "Life is green, but logic is gray and lifeless." The poets tell us that "we murder to dissect." Many believe that one’s time would be better spent in prayer, protesting, or preaching. Or if they are naturalistically minded, they might suggest contemplating one’s navel, or the sunset, or performing experiments in laboratories. So why study logic? Perhaps if we understood what logic is, we could better answer the question.

What Is Logic? 

In elementary school, you studied such things as reading, writing, and arithmetic. These subjects are correctly regarded as basic to all further education: One cannot study history, botany, or computers without being able to read. Reading, writing, and arithmetic are the basics, the tools that permit one to study further, and also to drive, to shop, and to get a job. But could there be something more basic than the three basics? Something so obvious that most people do not see it, let alone study it? What is there in common between calculating, reading, and writing? The answer of course is thought. One must think in order to read and write. Thinking, just as everything else, is supposed to follow certain rules, if we are to think correctly. Sometimes we make mistakes in thinking. We jump to conclusions; we make unwarranted assumptions; we generalize. There is a subject that catalogues these mistakes, points them out so that we can recognize them in the future, and then explains the rules for avoiding mistakes. That subject is logic.

The Place of Logic 

Logic is not psychology. It does not describe what people think about or how they reach conclusions; it describes how they ought to think if they wish to reason correctly. It is more like arithmetic than history, for it explains the rules one must follow in order to reach correct conclusions, just as arithmetic explains the rules one must follow to arrive at correct answers.

Logic concerns all thought; it is fundamental to all disciplines, from agriculture to astronautics. There are not several kinds of logic, one for philosophy and one for religion; but the same rules of thought that apply in politics, for example, apply also in chemistry. Some people have tried to deny that logic applies to all subjects, for they wish to reserve some special field—theology and economics, to name two historical examples—as a sanctuary for illogical arguments. What results is called polylogism—many logics—which is really a denial of logic.

But in order to say that there are many different sorts of logic, one must use the rules of the logic there is. Let those who say there is another kind of logic express their views using that other logic. It’s as though one were to claim that there are two (or more) sorts of arithmetic—the arithmetic in which two plus two equals four, and a second arithmetic in which two plus two equals twenty-two. Anyone who disparages or belittles logic must use logic in his attack, thus undercutting his own argument. This can perhaps be better seen by specifically discussing one of the laws of logic. The

Laws of Logic 

The first law of logic is called the law of contradiction, but recently some people have begun to call it the law of non-contradiction—the two phrases refer to the same law. Aristotle expressed the law in these words: "The same attribute cannot at the same time belong and not belong to the same subject and in the same respect." The law is expressed symbolically as: "Not both A and not-A." A maple leaf may be both green and not-green (yellow), but it cannot be both green and yellow at the same time and in the same respect—it is green in the summer, yellow in the fall. If it is green and yellow at the same time, it cannot be green and yellow in the same respect; one part, however small, will be green, another yellow. Greenness and notgreenness cannot at the same time and in the same way belong to a maple leaf.

To suggest another example: A line may be both curved and straight, but not in the same respect. One portion of it may be curved, another portion straight, but the same portion cannot be both curved and straight.

The law of contradiction means something more. It means that every word in the sentence "The line is straight" has a specific meaning. The does not mean any, all, or no. Line does not mean dog, dandelion, or doughnut. Is does not mean is not. Straight does not mean white, or anything else. Each word has a definite meaning. In order to have a definite meaning, a word must not only mean something, it must also not mean something. Line means line, but it also does not mean not-line—or dog, sunrise, or Jerusalem.

If line were to mean everything, it would mean nothing; and no one, including you, would have the foggiest idea what you mean when you say the word line. The law of contradiction means that each word, to have a meaning, must also not mean something.

Logic and Morality 

What do this law and the rest of logic have to do with morality? Simply this: When the Bible says, You shall not covet, each word has a specific meaning. Attacking logic means attacking morality. If logic is disdained, then the distinctions between right and wrong, good and evil, just and unjust, merciful and ruthless also disappear. Without logic, God’s words, "You shall do no murder," really mean: "You shall murder daily" or "Stalin was Prince of Wales." The rejection of logic means the end of morality, for morality and ethics depend on understanding. Without understanding, there can be no morality. One must understand the Ten Commandments before one can obey them. If logic is irrelevant or irreligious, moral behavior is impossible, and the "practical" religion of those who belittle logic cannot be practiced at all.

Something even worse, if anything could be worse, follows from rejecting logic. If logic does not govern all thought and expression, then one cannot tell true from false. If one rejects logic, then when the Bible says that Jesus suffered under Pontius Pilate, was crucified, dead, and buried, and rose again the third day, these words actually mean that Jesus did not suffer, was not crucified, did not die, was not buried, and did not rise again—as well as that Attila the Hun loved chocolate cake and played golf. The distinctions between true and false, right and wrong, all disappear, for there can be no distinctions made apart from using the law of contradiction.

The rejection of logic has become very popular in the twentieth century. In matters of morality, one frequently hears that "There are no blacks and whites, only shades of gray." What this means is that there is no good or evil; all actions and alternatives are mixtures of good and evil. If one abandons logic, as many people in this century have, then one cannot distinguish good from evil— and everything is permitted. The results of this rejection of logic—mass murder, war, government caused famine, abortion, child abuse, destruction of families, crimes of all sorts—are all around us. The rejection of logic has led—and must lead—to the abandonment of morality.

In matters of knowledge, we’re told that truth is relative; what’s "true" for you might not be "true" for me. So 2 plus 2 might be 4 for you and 6.7 for me. If logic is abandoned, then that also follows. Christianity is "true" for some—Buddhism is "true" for others. One result has been a growing antipathy toward Christianity, which claims that all men, not some, are sinners; and that there is only one way to God, through belief in Christ. Absolute truth— which is really a redundant phrase—has been replaced by relative truth, which is really a contradiction in terms (like square circle). But once logic is gone, truth is also. The use of logic is not optional. Logic is so fundamental, so basic, that those who attack it must use logic in order to attack logic. They intend the words "Logic is invalid" to have specific meanings. The opponents of logic must use the law of contradiction in order to denounce it. They must assume its legitimacy, in order to declare it illegitimate. They must assume its truth, in order to declare it false. They must present arguments if they wish to persuade us that argumentation is invalid. Wherever they turn, they are boxed in. They cannot assault the object of their hatred without using it in the assault. They are in the position of the Roman soldier who arrested Christ, but they do not realize, as the soldier did, that their position and action are dependent upon rules that they reject. They must use the rules of logic in order to belittle logic; he had to be healed by Christ before he could proceed with the arrest.

The Bible and Logic 

In the first chapter of the Gospel of John, John wrote, "In the beginning was the Logos, and the Logos was with God, and the Logos was God." The Greek word Logos is usually translated Word, but it is better translated Wisdom or Logic. Our English word logic comes from this Greek word logos. John was calling Christ the Wisdom or Logic of God. In verse nine, referring again to Christ, he says that Christ is "The true light" who lights every man. Christ, the Logic of God, lights every man. Strictly speaking, there is no "mere human logic" as contrasted with a divine logic, as some would have us believe. The Logic of God lights every man; human logic is the image of God. God and man think the same way—not exactly the same thoughts, since man is sinful and God is holy, but both God and man think that two plus two is four and that A cannot be not-A. Both God and Christians think that only the substitutionary death of Christ can merit a sinner’s entrance into Heaven. The laws of logic are the way God thinks. He makes no mistakes, draws no unwarranted conclusions, constructs no invalid arguments. We do, and that is one of the reasons why we are commanded by the Apostle Paul to bring all our thoughts into captivity to Christ. We ought to think as Christ does—logically.

Why Study Logic? 

To return to our first question, Why study logic? Our first answer must be that we are commanded by Scripture. Without learning how to think properly, we shall misunderstand Scripture. Peter warns against those who twist the Scripture to their own destruction. A study of logic will help us avoid twisting the Scripture and trying to make it imply something it does not imply. The Westminster Confession, written in England in 1648, says all things necessary for our faith and life are either expressly set down in Scripture or may be deduced by good and necessary consequence from Scripture. It is only through a study of logic that we can distinguish a valid deduction from an invalid deduction. But logic is indispensable not only in reading the Bible, but also in reading history, botany, or computer programs. It is applicable to all thought, and mistaken arguments maybe found in every subject. The study of logic will help us understand all other subjects better, not just theology. Therefore, as God said through the prophet Isaiah, Come, let us reason together.