A definitional argument is any argument in the form "X is a Y.", typically carrying the implication (sometimes stated explicitly) that therefore some set of rules should apply to X because of its membership in Y.
Debate involving such arguments tends to deteriorate because crucial pieces of information are missing.
Restating the argument more formally:
- Item X is an example of class Y.
The often-unstated part of the argument:
- We apply rule Z to all examples of class Y.
- ∴ Therefore rule Z should be applied to item X.
Once the argument is presented formally, it becomes clearer that we are missing the following pieces of information:
- What are the defining attributes of class Y?
- ...and which specific attributes of item X meet this definition, if it is not obvious?
- What is rule Z?
- Why is rule Z applied to all examples of class Y?
The original (common) form of the argument is generally not resolvable in a rational way because of these missing pieces of information, about which the arguers may be making completely different assumptions.
Agreement on the missing pieces of information has at least the potential to be reached by examining objective evidence (or by breaking them down into further resolvable arguments), thus leading to a resolution of the argument overall.
Argument: A U-Haul is a truck. Trucks are required to stop at highway weigh-stations. Therefore U-Hauls must stop at highway weigh-stations.
Every part of this example is true, but the conclusion is wrong; U-Hauls are not required to stop at weigh stations.
Although called "trucks" by almost everyone (including U-Haul), U-Haul trucks only have 2 axles, and the definition of "truck" which requires stopping at weigh-stations includes the attribute "3 or more axles". The reason for this is presumably that axle quantity is a good way of distinguishing between retail-level commercial vehicles such as U-Hauls and heavier bulk-commercial vehicles such as 18-wheelers which cause heavy wear-and-tear on highways and are therefore taxed by weight to help pay for repairing the extra road damage they cause.
Thus if someone built a super-heavy truck which nonetheless had only 2 axles, we might have to change the definition of "truck which must stop at weigh-stations" because the old definition would no longer work to meet our goal of equitably paying for pavement wear-and-tear.
Argument: Atheism is a religion.
In this case, the speaker is generally implying that some rule (or set of rules) which are normally applied to religions (and those who believe in them) should also apply to atheism and atheists. What never seems to be mentioned are the following missing pieces of information: (a) what are the defining attributes of a "religion"?, (b) what rule is the claimant arguing should be applied to atheism/atheists? and possibly (c) why is this rule applied to religions?