I trust it's obvious why this presumption looks as it is supposed to - "the bad consequence IS a means to the good consequence"?
Consider Herring's example on p. 169 with the surgeon. If the bad consequence (taking someone's heart out) is NOT a means to the good consequence (saving her), the cardiosurgeon looks negligent and unscrupulous! Why take her heart out?
Herring, Criminal Law: Text, Cases, and Materials (8 edn, 2018). p. 168
The ‘pure intention’ view
This view154 is promoted by those who argue that intention should mean purpose, nothing more and nothing less.155 Supporters point out that it is quite possible to foresee a result as virtually certain but not intend it. You may foresee that by drinking ten dry sherries in the bar you will get a hangover the next morning, but that does not mean that you intend to get a hangover when you drink.156 John Finnis has written that he may foresee that his lecture will confuse half those attending, but that does not mean he intends them to be confused.157 As Wilson points out: ‘If I pull my child’s loose tooth out, knowing this cannot be done without him suffering significant pain, only a passing spaceman from Mars could reach the
conclusion that I intended to hurt him.’ Although if Wilson is correct in saying this quite a few lecturers in criminal law are Martians!
Looking at our scenarios there is no doubt that under the ‘pure intention view’ the plane bomber, the burning father, and the revengeful wife did not intend the death of V.158 More problematic is the desperate surgeon. You might think that the pure intention view would hold that the surgeon’s purpose was to save his wife’s life and so he did not intend to kill V. However, the argument ‘I intended to take V’s heart out, but did not intend to kill her’ is very unattractive. One possible escape route for supporters of the pure intent view is to say that intending to take someone’s heart out simply is intending to kill them: the two consequences are inseparable.159 There is, in our world, no difference between taking out someone’s heart and killing them.
Supporters of the pure intention view often support the ‘ethical doctrine of double effect’, which has been defined by John Keown in the following extract:
J. Keown, Euthanasia, Ethics and Public Policy (Cambridge: Cambridge University Press, 2003), 20
According to this ethical tradition, it is permissible to allow a bad consequence to result from one’s actions, even if it is foreseen as certain to follow, provided certain conditions are satisfied. Those conditions are identified by the principle of ‘double effect.’ According to this ethical principle, it is permissible to produce a bad consequence if:
• the act one is engaged in is not itself bad;
• [2.] the bad consequence is not a means to the good consequence;
• the bad consequence is foreseen but not intended; and
• there is a sufficiently serious reason for allowing the bad consequence to occur.
It should be noted that Keown’s explanation of the doctrine of double effect is more than a description of mens rea. The fourth factor requires the jury to make a moral assessment of what the defendant did.160