The Wheels on the Bus, the Plane, and the End of the Argument

Article 1 of 10 — No Parachute: Following a Doctrine to Its Final Destination

“The wheels on the bus go round and round.”

A bus is a forgiving picture of an argument. It stops every few blocks. Its doors open. A passenger who dislikes the neighbourhood can step onto the pavement and take another route.

An airplane is less accommodating. Before takeoff, a passenger may refuse to board. Once the aircraft is aloft, he cannot open a door at thirty thousand feet merely because he dislikes the city printed on his ticket. He may object to the flight plan or insist that he intended to travel elsewhere. Unless the route changes, the aircraft continues toward the same runway. Protest is not a parachute.

Greg Bahnsen’s first lecture in Practical Apologetics gives four questions for testing an argument: Is it arbitrary? Is it inconsistent? What are its consequences? What are its preconditions? He later explains reductio ad absurdum: whatever implies what is false must itself be false. The airplane image captures that discipline, though it is used here as an illustration rather than offered as a verbatim quotation from Bahnsen.

Claims travel. Some travel farther than their advocates have noticed.

A Doctrine Has a Destination

Suppose a Christian confesses that God is true while also holding a theory which entails that a promise of God failed. His profession and his theory must be distinguished before either is judged.

His confession tells us what he consciously affirms. It may be sincere, well informed, and joined to an otherwise orthodox life. The consequence of his theory is a different question. If the theory cannot be true unless a genuine divine promise failed, sincerity cannot make the propositions agree. It may explain why he refuses the conclusion. It may show that he would abandon the error if he understood it. It does not alter the relation between premise and consequence.

The exact judgment is therefore stronger and fairer than a personal label:

This doctrine is atheistic in its necessary consequence, although its advocate expressly professes Christian theism and may not recognize the contradiction.

Calling the man an atheist would collapse profession, implication, knowledge, intention, and culpability into one accusation. Declaring the doctrine harmless because he professes belief in God would commit the opposite error. The proposition is what the argument can place on trial.

An inconsistent Christian is possible. God often preserves believers from following every error to its terminus. We may be thankful for that inconsistency while still refusing to protect the doctrine that produced it.

Not Every Road Is an Entailment

The word leads is cheap. A writer says that a doctrine “leads to liberalism” when he means that several liberals have held it. Another says that an idea “leads to unbelief” when he means that it creates a temptation. Historical association, psychological tendency, probable effect, and logical consequence are not the same.

A conclusion is entailed when the premises cannot all be true while the conclusion is false. That does not prove that the premises are true. It means that, once they are established and their terms remain fixed, the conclusion is no longer optional.

Consider the route at the centre of this series:

1. Every genuine promise of God is certain to be fulfilled.
2. God genuinely promised a particular result.
3. A disputed theory requires that the result did not occur.
4. Therefore, the theory requires the failure of a genuine promise of God.

The fourth statement follows only if the first three are true. Perhaps the alleged promise is being misread. Perhaps it concerns something other than the critic claims. Perhaps the theory does not deny its fulfilment. Any of those objections, if proved, breaks the route.

This is why severe conclusions require unusually clean proofs. Resemblance to unbelieving thought is insufficient. The use of historical evidence is not naturalism. A dangerous tendency is not a necessary consequence. A critic who hides a doubtful premise beneath a bold arrow has drawn a false flight path.

When the premises are established, however, intention has no power to cancel the conclusion. “I did not mean that” may be important evidence about the speaker. It is not an answer to the inference.

Scripture Follows Premises to Their Consequences

Necessary consequence is not a foreign judge placed above Scripture. The Westminster Confession says that the whole counsel of God is either expressly written in Scripture or “by good and necessary consequence may be deduced from Scripture.” The reasoning is ministerial: it receives what God’s words commit us to believe.

Christ reasoned this way against the Sadducees. He cited God’s covenant declaration, “I am the God of Abraham, and the God of Isaac, and the God of Jacob,” then drew its implication: “God is not the God of the dead, but of the living” (Matthew 22:32). The Sadducees denied the conclusion, but Christ showed that their own Scriptures required it.

Paul does the same in 1 Corinthians 15. If there is no resurrection of the dead, then Christ is not risen. If Christ is not risen, preaching is vain, faith is vain, the apostolic witnesses are false, believers remain in their sins, and the dead in Christ have perished. The Corinthians may not have intended to destroy the gospel. Paul followed their premise to the end anyway.

He did not confuse confused church members with conscious enemies of Christ. He exposed what their claim required so that they would abandon it. Consequence served correction.

The same discipline belongs in disputes over Scripture. A teacher may affirm divine truthfulness in one paragraph and advance a preservation theory incompatible with it in the next. The orthodox paragraph does not control the contradiction by courtesy. One of the claims must yield.

Inspect the Flight Plan

Before saying that a doctrine lands in atheism, five questions must be answered.

1. What exact proposition boarded the plane? A broad label such as modern textual criticism or KJV-Onlyism is not a premise. State the claim in words its advocate could recognize.
2. Which premises fix the route? Define the important terms and print every indispensable step. If the case depends upon a reading of Matthew 5:18 or the meaning of “kept pure,” make that burden visible.
3. Are the connections necessary? “This sounds like” and “people who say this often say that” cannot bear an atheistic verdict.
4. Where does the argument land? If the theory entails that God spoke falsely, explain what this means for the identity of the God who cannot lie and for the gospel resting upon his testimony.
5. Which premise must be surrendered? A useful reductio shows the road home. The defender may retract the theory, disprove a connecting premise, expose an equivocation, or show that the inference is invalid.

What he cannot do is affirm the proposition, concede the definitions and connecting premises, and veto the conclusion.

The purpose of this series is not to discover an elegant way to call Christians atheists. It is to prevent an orthodox profession from sheltering a premise that contradicts it. There is mercy in tracing the route before the error settles into a system.

The wheels on the bus may continue going round and round. A sound argument does not circle harmlessly because its passenger dislikes the terminal. You may refuse the premise before takeoff. You may prove that the proposed flight path is wrong. You may not remain aboard the argument and choose a different destination.

Sources and Notes

• Greg L. Bahnsen, “Arbitrariness (1 of 5),” Practical Apologetics, especially 8:31 on consequences and 1:11:09 on reductio ad absurdum. See the timestamped lecture outline and lecture recording.
• J. C. Beall and Greg Restall, “Logical Consequence,” Stanford Encyclopedia of Philosophy, revised May 17, 2024.
• The Westminster Confession of Faith (1647), 1.6.
• Scripture quotations are from the Authorized Version.