ABSTRACT
This chapter focusses on the medieval philosopher and theologian Thomas Aquinas, and his discussion of the admissibility of killing the aggressor in an act of self-defence. In the Secunda Secundae of his Summa Theologiae Thomas explicitly says that one human act can have two consequences: (i) one is good (saving one’s life) and (ii) the other is bad (aggressor’s death). If killing the aggressor is not included in the agent’s intention (is praeter intentionem) and is further appropriate to the act’s end (self-defence), it is not illicit.
But not all authors have regarded Thomas’s treatise as the first formulation of a certain version of the principle of double effect. Interpretative disputes have focussed on the meaning of the phrases in intentione and praeter intentionem and also per accidens and quandoque. I analyze the individual interpretations, in turn, and these analyses result in the claim that Thomas Aquinas in fact presents his version of the principle of double effect, which is, however, an application of already existing principles of moral evaluation of human action to the particular example of self-defence. His version of the principle of double effect therefore plays a fairly marginal role in his thought.
