Dynamic Priorities
In the previous examples involving priorities over arguments, we assumed
that priorities were given. However, even priorities may be determined
by (defeasible) arguments. Usually, a conflict between competing
arguments is adjudicated according to the comparative strength of the
defaults included in the such arguments. Therefore, priority arguments
aim to establish the comparative strength of such defaults. In the legal
domain, where legal norms provide the relevant defaults, priority
arguments may appeal to formal legal principles — i.e., criteria which
do not refer to the content of the norms at issue— such as the
preference accorded to the more recent laws (lex posterior derogat
legi priori ), to the more specific ones (lex specialis derogat
legi generali ), or to those issued by a higher authority (lex
superior derogat legi inferiori ). Priority arguments may also be
supported by textual clues, e.g., norms having negative conclusions are
usually meant to override previous norms having the corresponding
positive conclusions. Finally, priority arguments may refer to the
substance of the norms at issue, e.g., assigning priority to the norm
that promotes the most important values (legally valuable interests) to
a greater extent.
One way to deal with the argumentative role of priority arguments
consists in extending the IN and OUT labelling to defeat links between
arguments. The previous rules can then be rewritten as follows:
-
An argument \(\mathcal{A}\) or a defeat link \(\mathcal{d}\) is IN iff
no argument which is IN defeats \(\mathcal{A}\) or \(\mathcal{d}\)
through a defeat link which is IN.
-
An argument \(\mathcal{A}\) or a defeat link \(\mathcal{d}\) is OUT
iff an argument which is IN defeats \(\mathcal{A}\) or \(\mathcal{d}\)
through a defeat link which is IN.
We need to specify when a defeat link is defeated: an argument
\(\mathcal{A}\) defeats the defeat-link \(\mathcal{d}\) from argument
\(\mathcal{B}\) to argument \(\mathcal{C}\) when \(\mathcal{d}\) denotes
a rebutting attack from \(\mathcal{A}\) to \(\mathcal{C}\) and
\(\mathcal{B}\) prevails over \(\mathcal{C}\)
To clarify this idea let us return to the issue of the admissibility of
lying to save a person’s life. Let us now add a priority argument
(C ), stating that, since the statement that Bob is away will save
Bob’s life, the duty to make the statement, as supported by the argument
from good consequences, outweighs the duty not to make it, as supported
by the prohibition on lying (Figure 15).