What is Logic and Why Should Christians Use It?


The art of using Reason* well in our inquiries after truth, and the communication of it to others. *Reason – includes all of the intellectual attributes of man. – Isaac Watts.  Logic

Logic is the study of the methods & principles used to distinguish correct from incorrect reasoning. – Copi & Cohen. Introduction to Logic 12th Ed.

Logic is the study of argument. – Engel. With Good Reason 6th Ed.

Logic prescribes how to combine concepts into judgments & judgments into syllogisms & chains of reasoning so as to achieve truth. – Sister Joseph. The Trivium.

Logic is, indeed, the science of human reasoning, or the study of the rules, laws, means, and ways humans should think. – McDurmon. Biblical Logic.

Logic, then, is the science which studies the general principles in accordance with which we think about things, whatever things they may be; and so it presupposes that we have thought about things. – Joseph. An Introduction to Logic.

Logic, says J. S. Mill, in the Introduction to his work, is common ground on which the partisans of Hartley and of Reid, of Locke and of Kant may meet and join hands.

Logic is concerned primarily with (1) validity of arguments and (2) consistency of propositions. – Dr. John Frame

As you can see Logic affords us as many definitions as there are definers. But from all of these we can reduce it down to a simple definition for our use. Logic is the study of correct reasoning and argumentation as opposed to incorrect reasoning and argumentation. Correct and incorrect imply an objective standard for truth. Such an objective standard must be absolute, invariant, transcendent and yet immanent. The only standard that meets these conditions is the Bible. For Logic, to be of any use and have any “authority”, it cannot have its foundations in the subjectivism of autonomous human reasoning. It must exists above and apart from human reasoning and must be an expression of God himself. In my previous post, The Laws of Logic as an Expression of God’s Character, I demonstrated that the 4 Laws of Logic have their origins in God’s way of thinking. Because Logic has its foundation in God’s way of thinking, it is universal, extending to all spheres of human endeavor. Because how God thinks is expressed in the Bible, we can, and in fact are commanded to, conform our way of thinking to God’s way of thinking expressed in the Bible. This allows us to use the toolbox of Logic to investigate and come to the knowledge of truth in all areas of human reasoning.

Why Should Christians Use Logic?

The Cultural Mandate.

How can we subdue creation to the glory of God, without knowing how to reason? In carrying out the cultural mandate logic becomes indispensable. If we don’t know how to reason properly, which is the realm of logic, how are we to carry out the cultural mandate? The cultural mandate calls us to think logically. Mathematics, the language of science, has logic as its foundation. Without logical reasoning science would become impossible and we would fall back into the Dark Ages of myth and superstition. The Reformation, with it’s emphasis on Sola Scriptura, led mankind out of superstitious darkness into the light of sound, Biblical reasoning. With the supremacy of the Bible as the foundation of reasoning, the sciences grew. In cultures were the Bible is not the foundation for reasoning, science is stillborn. For a modern example of this we only have to look at the violent 7th century religion of death, known as Islam. Though muslims had some scientific advances at times, the overall worldview of Islam made the scientific advances of the west, due to our Christian worldview, impossible for the muslims.

Understanding the Bible.

Logic, as a tool, is indispensable for understanding the Bible. Though the Bible does not tell us every theological truth, it does tell us various truths and from these truths we are expected to draw conclusions. The Doctrine of the Trinity is a clear example of this. Though not explicitly stated in scripture, the Bible does provide us verses that logically lead to the Doctrine of the Trinity. In order to do theology we must use logic. The Creeds, Confessions, & Catechisms of Christianity demonstrate the necessity of using logic to clearly expound the truths of scripture. Reasoning from scripture to true doctrines would be impossible without logic.

Obeying God.

In order to obey God, the Christian must use logic. Let’s take the commandment, Thou shalt not commit adultery. That woman is not my wife. That woman is another man’s wife. God has said I am not to have sexual relations with any woman that is not my wife. Therefore I should not have sexual relations with her. To think ethically, one must think logically.

Defending the Faith.

In the apologetic encounter logical thinking helps us to expose the internal inconsistencies within the unbelievers worldview. The laws of logic can even be used as a demonstration of God’s existence. For the laws of logic to be useful, they must be universal, invariant, and transcendent. This implies that the laws of logic cannot be the creations of autonomous man, but must come from a mind that is also universal, invariant, and transcendent, namely God’s mind.

Because God Commands Us to Reason

Isaiah 1:18 Come now , and let us reason together , saith the LORD:

God engages in reasoning and commands us to reason. To follow his pattern of thought we should follow the way God reasons.

Hebrews 6:18 That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:  

God does not utter one proposition and then utter a contradictory proposition. Our thinking should also be consistent and therefore modeled around God’s character.

2 Corinthians 1:18 But as God is true, our word toward you was not yea and nay.

Paul tells us that his word was not contradictory. He didn’t affirm a proposition and then deny the same proposition. What was Paul’s foundation for this? God is true. Therefore we should not be contradictory in what we say, because God is true.

Matthew 5:37 But let your communication be , Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.

Here again scripture, through the words of Jesus, instructs us to be consistent in what we say. Jesus informs us that not being consistent in what we say “cometh of evil.”

Matthew 6:24 No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.

Again Christ instructs us on the necessity of consistency.

2 Corinthians 1:12 For our rejoicing is this, the testimony of our conscience, that in simplicity and godly sincerity, not with fleshly wisdom, but by the grace of God, we have had our conversation in the world, and more abundantly to you-ward.

Scripture instructs us through Paul’s example that our actions are to be free of inconsistency and hypocrisy. Because God is consistent, our words must be consistent. Some Uses of Reasoning in the Bible.

Matthew 16:2 He answered and said unto them, When it is evening, ye say , It will be fair weather: for the sky is red . 3 And in the morning, It will be foul weather to day: for the sky is red and lowring . O ye hypocrites, ye can discern the face of the sky; but can ye not discern the signs of the times?

Jesus calls on the hearers to use their reasoning ability.

Mark 2:6 But there were certain of the scribes sitting there, and reasoning in their hearts, 7 Why doth this man thus speak blasphemies? who can forgive sins but God only? 8 And immediately when Jesus perceived in his spirit that they so reasoned within themselves, he said unto them, Why reason ye these things in your hearts?

Jesus challenges the reasoning process of the scribes.

The Bible Shows the Necessity of Reasoning.

Acts 17:17 Therefore disputed he in the synagogue with the Jews, and with the devout persons, and in the market daily with them that met with him.

Acts 19:8 And he went into the synagogue, and spake boldly for the space of three months, disputing and persuading the things concerning the kingdom of God. 9 But when divers were hardened, and believed not, but spake evil of that way before the multitude, he departed from them, and separated the disciples, disputing daily in the school of one Tyrannus.

2 Corinthians 10:4 (For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;) 5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;

Our thinking must be logical in order to dispute and persuade unbelievers and in order to disciple our thinking and bring it under the obedience of Christ. It is through logical reasoning that we are able to cast down all imaginations that exalt themselves against the knowledge of God.

1 Peter 3:15 But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear:

Logical thinking is a necessary in order to obey God in providing a reason of the hope that is in us.

Theological reasoning exemplified in Bible.

Implication Hypothetical argumentation

Mark 2:27-28  27 And he said unto them, The sabbath was made for man, and not man for the sabbath: 28 Therefore the Son of man is Lord also of the sabbath.

If-Then argumentation

Rom 4:2 For if Abraham were justified by works, he hath whereof to glory; but not before God.  

Rom 5:15 But not as the offence, so also is the free gift. For if through the offence of one many be dead , much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many. 

1 Corinthians 15:13 But if there be no resurrection of the dead, then is Christ not risen: 14 And if Christ be not risen, then is our preaching vain, and your faith is also vain. 15 Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up , if so be that the dead rise not. 16 For if the dead rise not, then is not Christ raised: 17 And if Christ be not raised, your faith is vain; ye are yet in your sins.   

From the underlined above we can demonstrate modus ponens

  1. If Christ is risen, then your faith is not vain.
  2. Christ is Risen.
  3. Therefore your faith is not vain.

The Westminster Shorter Catechism has as its answer for the first question:  The chief end of man is to glorify God and fully to enjoy him forever.  Logic is not just a secular area of thought, but it is an area in which we as Christians should lead the way.  We glorify God when we think logically, because we exemplify God’s way of thinking in logical thinking.  Is logic a neutral tool? No, but it doesn’t mean that the laws of logic are different unbeliever and believer. The use of logic isn’t neutral. Should the study of logic cause concern for Christians?  If we know the character of God as revealed in Scripture, then we have a clear Biblical justification for the use of logic.

In the next post I will cover the fundamental terms in logic and their usage.

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