At one point in the classic book The Lion, the Witch and the Wardrobe by C.S. Lewis, an exasperated professor utters the words, "Logic! Why don't they teach logic at these schools?" 1Lewis not only believed in truth, but also in our ability to use logic as an aid in determining truth.
Although it is true that logic studied as a formal discipline can lead to challenging formulas and diagrams, in a basic sense we all use it regularly. In reading this article, for example, you are using logic to interpret the words you see. Without logic, you could not make sense of this sentence. Interpreting a rational sentence requires a rational mind with the ability to comprehend words that are structured in a way that makes sense — in short, words that are organized logically.
Historically, Aristotle was the first to explore logic as a formal discipline, but he did not invent it. The underlying principles of logic are readily discernible. We use them every day to get through some of the simplest decisions and actions in life.
Logicians generally offer four broad principles or laws of logic.
First, the law of identity makes the obvious observation that something is itself and, therefore, cannot be something else (A is A).
Second, the law of non-contradiction deals with the concept of antithesis and states that something cannot be true and not true at the same time and in the same sense (A is not non-A).
Third, the law of excluded middle is often presented as "either A or non-A." For instance, God either exists or He does not exist.
Fourth, the law of bivalence assesses propositions as either being true or false. "God exists" is either a true or a false statement. 2
Furthermore, the Bible is not against logic. In fact, biblical Christianity encourages the use of the mind. In Isaiah 1:18 (NIV) we read, "'Come now, let us reason together,' says the LORD." In the New Testament Jesus is clear that we are to love God — not only with all our heart, soul, and strength, but also with our mind (Matthew 22:37; Mark 12:30; Luke 10:27).
Christians are also called to defend the truth by appealing to reason and evidence (Acts 26:25; 1 Peter 3:15). Acts 1:3, for example, says Jesus "gave many convincing proofs" as evidence for His resurrection. In Acts 26:25-26, after the Apostle Paul gives his testimony and outlines the gospel message, he is accused of being insane. Paul replies: "What I am saying is true and reasonable. The king is familiar with these things, and I can speak freely to him. I am convinced that none of this has escaped his notice, because it was not done in a corner."
In Acts 26, Paul uses logic to make his case for Christianity. He appeals to his own experiential testimony (noting how he used to persecute Christians), makes mention of the resurrection of Christ, and implies that many are aware of the events surrounding the gospel of Christ. Logically, Paul believes the Christian message because of the evidence, both experiential (his encounter with Christ) and evidential (the case for the resurrection, for instance, and the testimony of witnesses).
First Peter 3:15 also appeals to logic: "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have." The Greek word translated as "answer" is apologia and was used in reference to giving a legal defense. In this passage, Peter calls readers "to give the reason for the hope that you have." Reasons presuppose the validity of reason and logic.
The appeal in Acts 1:3 to "many convincing proofs" again relates to logic because a reasonable appeal is made to the evidence for the resurrection. In other words, Luke, the author of Acts, is not asking for blind faith, but faith founded on logical inferences. The New Testament records that many people saw Christ after his death, that Christ proved he was raised bodily by eating food and by inviting doubting Thomas to touch him (John 21:12-13; Luke 24:38-43). These are all logical appeals to evidence and reason.
As a former atheist, C.S. Lewis was well aware of the role that reason played in his conversion to Christianity. That's why he offered reasonable, logical arguments in support of his beliefs. In doing so, Lewis often utilized what is known as abductive reasoning. Abductive reasoning is similar to reasoning used by the scientific community in that it uses reasonable evidence to come to the best explanation.
In making his case for Christianity, Lewis used abductive reasoning to argue that the Christian explanation of reality — the Christian worldview — is more reasonable and probable than the alternatives.
In part two of this article, I describe two of Lewis' key arguments that use abductive reasoning: the argument from Christ and the argument from longing.