By SHIDARTA (July 2026)

In the study of legal reasoning, there is material concerning methods of legal discovery (rechtsvinding), encompassing interpretation and construction. We are familiar with various interpretive methods such as grammatical, authentic, systematic, comparative, teleological, and futuristic interpretations, among others. We have also long been acquainted with construction methods such as argumentum per analogiam (analogy), argumentum a-contrario, and argumentum a-fortiori. However, several methods are rarely discussed, or are not even categorized under the ranks of interpretation or construction. One of them is argumentum ad absurdum. It is not entirely incorrect to classify argumentum ad absurdum as a second-order method of legal discovery because its function is to evaluate the methods presented at the primary level. This designation is given because it does not generate new norms; rather, it is primarily intended to reject the outcome of a particular interpretation and/or construction.

Argumentum ad absurdum literally translates to an argument leading to absurdity. Typically, this mode of reasoning is utilized to demonstrate that a particular argument contains reasoning inconsistencies that ultimately yield an impossibility or oddity. In classical logic, argumentum ad absurdum is often referred to as reductio ad absurdum or proof by contradiction.

Is argumentum ad absurdum a logical fallacy? I do not categorize it as such. It differs from types of reasoning recognized as fallacious, such as argumentum ad hominem, argumentum ad populum, argumentum ad baculum, argumentum ad verecundiam, or argumentum ad misericordiam. It is worth noting that historically, the Latin phrase “argumentum ad…” means “an argument directed at….” Not every phrase phrased as “argumentum ad…” denotes a fallacy.

Thus, argumentum ad absurdum is not a logical fallacy. It is a valid argumentation technique. In formal logic, this constitutes a valid form of reasoning. The logical structure is as follows:

• If premise A is true, then consequence B will follow.
• B is impossible or absurd.
• Therefore, A must be rejected.

We can begin the explanation with an illustration. Suppose on a road leading to a residential complex, there is a traffic sign prohibiting all vehicles from passing through. This sign was installed by the local Transportation Agency. To enter the complex, all vehicles must detour via a different route. At one point, a fire breaks out in a house within that complex, located not far from the no-entry sign. A fire truck arrives at the scene, yet it is stopped and instructed by a traffic officer on duty to take the detour typically used by local residents. Although the fire truck eventually reaches the location, it takes about 25 minutes longer than if it had been permitted to bypass the prohibition sign.

How do we evaluate this situation? A legal reasoner must not initially leap to the conclusion that the “no-entry” rule is a bad rule. That is not where the problem lies! The rule becomes problematic because it was read without contextual exceptions. Interpreting the rule in such a manner obstructed another function far greater than merely prohibiting vehicles from entering: the imperative to rescue property and/or human lives from a fire disaster, which unequivocally warrants higher priority. The consequence of applying the no-entry rule without such exceptions produced an outcome that is legally unacceptable, rather than merely undesirable. Thus, argumentum ad absurdum is a method demonstrating that a premise must be rejected because, if consistently enforced, it produces consequences that are absurd, irrational, contradictory, or impossible to have been intended by the legislature.

We can say that the focus of argumentum ad absurdum is not merely that “the function of the norm is impaired,” but rather that “the outcome of such reasoning leads to consequences that are unacceptable to the legal system.” In other words, what argumentum ad absurdum tests is the logical consequence of a particular mode of reasoning. This consequence often emerges precisely when someone has already applied another method beforehand. In the example above, argumentum ad absurdum arises when the traffic officer applies a grammatical interpretation to the no-entry rule.

Argumentum ad absurdum can also occur after applying systematic interpretation. Consider the following example. Suppose there are several interrelated statutory provisions regulating the same situation. Article 10, paragraph (1) of a statute states that all vehicles are prohibited from entering high-ranking state officials’ residential complexes. Paragraph (2) of that same statute states that “government official vehicles are exempted from the prohibition as referred to in paragraph (1).” Meanwhile, Article 25 of another statute provides that medical personnel are obligated to provide emergency assistance as swiftly as possible.

At one point, an emergency call is made requesting that a private hospital near the residential complex dispatch an ambulance immediately to pick up a patient inside. If the three provisions are read systematically, they produce the following understanding. First, Article 10, paragraph (1) prohibits all vehicles. Second, Article 10, paragraph (2) only grants an exemption to government official vehicles. Third, Article 25 of the other statute mandates that all medical personnel deliver emergency aid as swiftly as possible. In this event, the arriving ambulance is not a government official vehicle, but one from a private hospital. Because a private ambulance is not a government vehicle, it remains prohibited from entering the complex. Here, it is evident that systematic interpretation also culminates in an argumentum ad absurdum.

It may seem that argumentum ad absurdum occurs whenever the construction of a rule deviates from what was “intended by the legislature.” We must be cautious about defining it this way, as doing so leads us toward teleological interpretation (purposive interpretation).

Let us provide a follow-up illustration. Suppose the prohibition against non-governmental vehicles entering the high-ranking officials’ residential complex is aimed at maintaining maximum security. Consequently, it is unsurprising that CCTV cameras are installed at various corners of the complex. However, it turns out these CCTV cameras also extend into the private spaces inside every house. Furthermore, these recordings are required to be stored for at least one week (7×24 hours). The underlying rationale is that the more surveillance cameras there are, the safer the residents will be. All of these rules clearly reflect a teleological interpretation because they originate from the goal of maximum security.

However, this rule brings about the consequence that privacy in private quarters ceases to exist; everyone feels surveilled, and every activity is recorded over a certain duration. If the security objective is construed so broadly as to justify such excessive surveillance, the right to privacy practically vanishes altogether. Such a teleological interpretation leads to an absurd consequence, as the law is supposed to safeguard the privacy of its citizens, yet such a rule infringes upon that privacy. Thus, argumentum ad absurdum does not criticize the objective of “maximum security for high-ranking state officials.” The focus of its critique is the teleological outcome.

Can the outcome of a legal construction also be evaluated through argumentum ad absurdum? The answer is undeniably yes. For instance, there is a regulation prohibiting passengers from bringing sharp weapons onto a flight. An officer discovers a small safety pin on a passenger’s hat. While moving the hat from the tray at the X-ray machine, the officer accidentally pricks their finger on the pin, which happens to be open. The tip of this pin is indeed sharp! Because sharp weapons are prohibited, the sharp-tipped safety pin is deemed a weapon and thus barred from the cabin. This is the outcome of a construction by analogy. Consequently, no one would be permitted to carry a safety pin to secure their hat or a loose sarong; even women wearing hijabs would be prohibited from carrying pins to adjust their veils. Construction through analogy in this instance produces an absurd consequence because it negates the normal function of commonplace daily activities. If this analogy were carried further, even a toothpick could be deemed a sharp weapon. Therefore, such a construction is untenable.

Thus, argumentum ad absurdum can be positioned as a method specifically employed to evaluate whether the outcome of legal reasoning is acceptable. As shown in the previous examples, we test whether the outcomes of grammatical, systematic, and teleological interpretations are acceptable. Argumentum ad absurdum offers evaluative reasoning. It is here that argumentum ad absurdum is situated, likely alongside arguments regarding coherence, proportionality, or systemic consistency. (***)

This article is a translation of the following Indonesian article: https://business-law.binus.ac.id/2026/07/06/argumentum-ad-absurdum/