Concept

Logic

Intro

Logic is the study of how good reasoning works. Some arguments lead from true starting points to a true conclusion. Other arguments look convincing but are quietly broken. Logic is the toolkit for telling the difference.

It rests on three rules so basic almost no one notices them. A is A (a thing is itself). A cannot be both true and not true at the same time and in the same sense. A statement is either true or false. Aristotle named these the three laws of thought, and every conversation, scientific paper, and courtroom argument depends on them, including arguments against them.

Logic matters for apologetics in three ways. First, every theistic argument (the cosmological argument, the moral argument, the resurrection argument) uses logical structure to do its work. A clear understanding of logic helps you set up a case properly and helps you see when somebody else's case is failing on the structure rather than on the facts.

Second, most atheist objections fail at the logical level rather than at the evidence level. "There is no truth" is a truth claim. "All beliefs are merely cultural" is a belief that claims to be more than cultural. "Reason and faith are opposites" is a faith claim about reason. The objections quietly contradict themselves; you only need to point it out.

Third, there is a deep apologetic move called the transcendental argument. It says the laws of logic themselves cannot be explained by a materialist worldview. Where do the laws come from? Why do they apply everywhere and forever? Why are they binding on minds? Theism has an answer: they are the structure of God's own reason, holding for the world He made. Atheism is stuck either calling them a useful illusion (which destroys reasoning itself) or smuggling in something that looks suspiciously like God.

This is the master hub for the logic topic. Subpages cover the three laws in detail, fallacies, formal vs informal logic, deduction vs induction vs abduction, and the apologetic uses of each.

Quick reply line: "Three rules: a thing is itself, no contradiction, every statement is true or false. Most atheist arguments contradict themselves on rule two. Most theistic arguments build forward from rule one. Logic is not neutral; the laws hold because mind is the bottom of reality."

In full

Layer-1 master hub for the philosophical discipline of logic, the study of correct reasoning, the structure of valid argument, and the formal principles governing inference. The folder holds the constituent topics; this page is the meta-orientation.

Logic matters for Christian apologetics because (a) every defeater and every theistic argument relies on logical structure to do its work, (b) atheist objections often fail at the level of logical structure (self-defeating, equivocal, ad hoc), and (c) the transcendental family of theist arguments (see Transcendental Argument for God, Stealing from God Argument) makes the case that the very laws of logic themselves require a theistic foundation, that logic is theistic in its presuppositions, not neutral between worldviews.

The three foundational laws

Classical logic rests on three foundational laws of thought (Aristotle, Metaphysics IV; sharpened by the medieval Scholastics and the 19th-century formal logicians):

Law of Identity, A is A. Everything is identical to itself. (Property of self-identity is necessary for any reasoning at all.)
Law of Non-Contradiction, A and not-A cannot both be true at the same time and in the same sense. No proposition is both true and false simultaneously. The foundational law that makes meaningful discourse possible.
Law of Excluded Middle, A or not-A; tertium non datur. Every proposition is either true or false; there is no third truth-value.

These three laws are non-negotiable in classical logic. Some non-classical logics (intuitionistic, paraconsistent) relax one or another, but the relaxation is highly technical and the classical laws remain the default for everyday reasoning, mathematics, and apologetic argument.

Logical argument forms

The folder contains entries for the major argument forms:

Reductio ad Absurdum, assume the opposite, derive a contradiction, conclude the original
Modus Ponens, if P then Q; P; therefore Q
Modus Tollens, if P then Q; not-Q; therefore not-P
Hypothetical Syllogism, if P then Q; if Q then R; therefore if P then R
Disjunctive Syllogism, P or Q; not-P; therefore Q
Constructive Dilemma
Inductive generalization (the move from observed to unobserved cases)
Abduction / inference to the best explanation (used in Minimal Facts Argument for the resurrection, fine-tuning arguments, etc.)

Fallacies, invalid reasoning

The sub-folder holds the catalog of informal-logic fallacies, the patterns of incorrect reasoning that get deployed frequently in atheist objections and apologetic conversations. The 22-entry catalog is complete and includes:

Ad hominem, attacking the person rather than the argument
Straw man, attacking a misrepresented version of the opponent's position
No true Scotsman, defining away counter-examples by redefining the category
Genetic fallacy, judging an idea by its origin rather than its content
Begging the question (petitio principii), assuming the conclusion in the premises
Equivocation, using a key term in two different senses across an argument
Composition / division, illegitimately moving from properties of parts to properties of wholes or vice versa
False dichotomy, presenting only two options when more exist
Affirming the consequent, if P then Q; Q; therefore P (invalid)
Denying the antecedent, if P then Q; not-P; therefore not-Q (invalid)
Tu quoque, "you too" deflection
Appeal to authority / popularity / consequences / ignorance / emotion
Slippery slope
Post hoc ergo propter hoc, temporal sequence mistaken for causation
Cognitive biases, Confirmation Bias, Availability Heuristic
Others, see Fallacies catalog

The catalog includes for each entry: diagnostic (how to spot it), rebuttal (how to answer it when deployed against you), and false-fallacy (when something looks like the fallacy but is actually a valid move, common with ad-hominem-vs-relevant-character-questions, slippery-slope-vs-actual-slope, etc.).

Logic and the transcendental argument for God

The presuppositional school of Christian apologetics (Cornelius Van Til, Greg Bahnsen, John Frame) deploys the laws of logic themselves as evidence for theism. The argument:

The laws of logic are universal (they hold everywhere), invariant (they do not change), abstract (they are not physical objects), and normative (they prescribe how reasoning ought to proceed).
On atheism / naturalism / materialism, there is no adequate account of why such entities should exist or have universal-invariant-abstract-normative character. (Materialism only acknowledges matter; conventionalism makes them parochial; evolutionary accounts make them contingent on biological history rather than universal-invariant.)
On Christian theism, the laws of logic are grounded in the eternal mind of God, they are necessary features of the divine intellect, with universal-invariant-abstract-normative character because they reflect God's own rational nature.
Therefore the laws of logic are evidence for theism, and more specifically, evidence for the Christian conception of God as the source of all rational order.

The Transcendental Argument for God develops this. The Stealing from God Argument (Frank Turek's popular-deployment form) shows the atheist borrowing logic, science, mathematics, ethics, etc. from the Christian worldview while denying the worldview that makes them possible. The atheist cannot do mathematics or logic as an atheist; the practice presupposes a framework atheism cannot supply.

This is one of the most distinctive and disputed Christian contributions to logic. Not all Christian apologists accept the presuppositional case (some prefer the classical evidential / cumulative-case approach, see Apologetic Method Comparison). But the argument is on the table and is deployed especially by the presuppositional school.

Apologetic deployment

When to deploy logic-specific arguments:

The interlocutor has made a formal error (affirming the consequent, equivocation, begging the question). Diagnose the fallacy, name it, redirect.
The interlocutor's position is self-defeating. Deploy Reductio ad Absurdum.
The interlocutor uses logic to attack Christianity. Deploy the transcendental move: ask where the validity of their logical inference comes from on their worldview.
The interlocutor denies logical principles (postmodernist relativism, etc.). The denial is self-refuting; deploy retorsion, the principle they deny must be assumed for the denial to be meaningful.

Don't deploy:

Against a wounded interlocutor for whom the logical conversation is intellectualization-defense (see Psychology of Lowered Defenses §3). The pastoral move precedes the logical one.
In a way that scores points without serving the gospel. Logic is a servant of evangelism, not its substitute.