A standard formulation of Double Effect allows doing an action that has a basic evil E as a consequence provided:
- The action in itself is neutral or good.
- E is not intended, whether as means or as end.
- There is a good G intended.
- The bad effects are are not disproportionate to the good effects.
In fact, if we take the proportionality condition simply to say that the overall consequences of the action are positive, then Double Effect will allow too-close variants of paradigm cases of what it is taken not to allow. For instance, a paradigm example of what Double Effect is taken not to permit is terror bombing in war. Terror bombing is a bombing intended to cause civilian casualties so as to terrify the enemy into surrender. That violates condition (2), since the evils are intended as a means to enemy surrender.
But now imagine that the person operating the bombs is an ethicist who believes in Double Effect with the consequentialist condition in place of (4). She realizes that knowledge is a good. In particular, it is good to know what it looks like from close up when civilian buildings have bombs dropped on them. So, she plans to drop bombs on civilian buildings to find out what that looks like. The action of dropping bombs in itself is neutral. (For instance, one might drop bombs as a means to mining.) She is pursuing a good G, of learning what it looks like when civilian buildings are bombed. She does not intend civilian deaths, either as an end or as a means: that there be civilians in the buildings does not contribute to her end, which is to see what it looks like when the buildings are bombed. So, (1)-(3) are satisfied. And if the bombing can be reasonably expected to end the war, thereby preventing further bloodshed, the overall consequences of the bombing can be assumed to be positive. But while this bomber is not intending civilian deaths, her variation on terror bombing is surely impermissible. Moreover, we see the pattern now: all that is needed for Double Effect with a consequentialist proportionality condition to justify a consequentialistically acceptable action is that the agent find some trivial good served by the action, and then the agent can act for that end. In other words, Double Effect ends up working like consequentialism with a bit of clever mental juggling.
So, the proportionality condition cannot be taken to be overall positive consequences. Maybe, though, there is a modification of the overall positive consequences criterion that works. Let C be the set of causal consequences of the action. At least one member of C is a basic evil. Let C* be the subset of C of those consequences c with the property that c does not have any basic evil in C as a necessary cause. Then, the modified consequentialist proportionality condition is that the overall value of C* is positive. This takes care of the above case, because the relevant increase in the probability of ending of the war has as a necessary cause the deaths of the civilians.
Incommensurability, however, precludes even this kind of consequentialist criterion. Also, I wonder if the above filtered consequentialist criterion isn't too restrictive. For instance, it wouldn't allow the defeater-defeater move I make in the second comment here, and it may be that theodicy requires such a move at some point.
All this suggests to me that the proportionality is a very complex notion. It may be one of those things that can't be codified (at least sufficiently briefly for us humans to do it in this life), but needs to be weighed by the Aristotelian phronimos.