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:
  1. 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.
  2. 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).