Levels of Abstraction of Arguments

Defaults can have different levels of abstraction, some representing general patterns of inference or inference schemes (Walton, Reeds, and Macagno 2008), others representing more specific connections between preconditions and conclusions. Indeed, the same conclusion can often be argued by using either a general inference scheme or a more specific rule. Consider, for instance, the issue of the morality of lying, which was the object of a famous controversy between Emmanuel Kant and Benjamin Constant (see Kant, I. ([1797] 1949). John shows up at Mary’s door at asks her whether Bob is at her place. Assume that Bob is in the house, that Mary is aware of this, and that Mary knows that John is armed and intends to kill Bob. The issue is whether Mary should lie, saying that Bob is away so as to save his life.
Let us first consider the argument according to which Mary should not lie. One way to frame this argument is as an argument pertaining to the implementation of moral rules in general. In that case, the premises of the argument could presented as follows:
  1. If rule “if P then Q ” is a moral principle, and P is the case, then presumably Q .
  2. The rule “if statement is a lie, then one should not make the statement” is a moral principle.
  3. The statement that Bob is away is a lie.
By defeasible modus ponens, these premises lead to the conclusion that
  1. Mary should not make the statement that Bob is away.
However, the argument can also be framed in a more specific way, taking the prohibition on lying for granted and using it directly as a premise:
  1. The statement that Bob is away is a lie.
  2. If a statement is a lie, then presumably one should not make the statement.
It seems to me that this second approach fits better our commonsense reasoning, in which we directly use the warrants we endorse to derive specific conclusions. Considerations pertaining to the foundation or the nature of such warrants are brought in through further arguments. For instance, the adoption of a warrant may be supported by arguments pointing to the consequences of its practice (e.g, in a rule-utilitarian perspective, the prohibition to lie may be supported by considering the benefit deriving from his generalised practice). Similarly, the strength and function of a warrant can be supported by arguments pointing to its nature (e.g. the fact that a principle pertains to morality may support its superiority over self-interested reasons, or or the fact that it belongs to the law may support its coercive enforceability or its exclusionary nature).
Again, by defeasible modus ponens, these premises lead to the presumable conclusion that
  1. Mary should not make the statement that Bob is away.
Let us now consider an argument why Mary should, on the contrary, lie. To build this argument we can appeal to a different pattern of defeasible inference: call it “inference from good consequences” or teleological argument. According to this pattern, the premises
  1. Making the statement that Bob is away will lead to the consequence that Bob will be saved (rather than being killed by John).
  2. This consequence is good.
  3. If an action has a good consequence then presumably we should do it.
lead to the conclusion that
  1. Mary should make the statement that Bob is away.
The two arguments and their conflict are represented in Figure 10.