Philosophers working on the ethics of war agree that proportionality is a key concept in both jus ad bellum and jus in bello. In this project, I aim to advance our understanding of this key concept, through four strands of work.

1. Proportionality under Uncertainty
How to make proportionality judgments under conditions of uncertainty is a key philosophical and practical question that has received almost no attention from philosophers. A new theory of proportionality under uncertainty will be developed.

2. The relationship between proportionality ad bellum and proportionality in bello
A traditional view (articulated most forcefully by Michael Walzer) held that our proportionality judgments about acts in war are fully independent from our judgments of the proportionality of the war overall. Many >revisionist< or >reductionist< scholars (following Jeff McMahan) have argued against this. Recent scholarship, therefore, has sought to tie the ad bellum and in bello principles together: the ad bellum assessment becomes the summing up of the individual in bello assessments. I will examine this reductionist view, and develop an account of how ad bellum and in bello proportionality inter-relate.

3. Proportionality in the Aggregate
In this strand of work I examine the issue of how to understand how proportionality works when we face multiple threats – in particular in choosing which of two (unconnected) threats to harm.

4. The inputs to proportionality calculations
Proportionality calculations demand comparisons. Broadly speaking, we must compare the amount of harm we will do with the amount of good we will do. In this strand of work I will question whether the inclusion of serious harms or large goods should exclude other harms and goods from being considered. Some work on >who to save< suggests that when very great goods are on offer, >irrelevant< minor goods should be excluded from consideration. I will probe this view, and see whether it can inform work on proportionality, where we consider not only who to save, but also who to harm, and how much. (Patrick Tomlin)