Isn't 2 (Duff's test of failure) just the contrapositive of 1? I can rephrase 2 as
2.1. If the result didn't occur, would D regard himself as having failed in his plan?
The logical contrapositive of 2.1 is
2.2. NOT(D regards himself as having failed in his plan) ⇒ NOT(the result didn't occur).
2.3. D regarded himself as succeeding in plan ⇒ Result did occur.
3. Does 2.3 change the author's example with Martin and Alfreda? Doesn't Martin fail 2.3 too, just like how he fails Duff's test of failure? Martin would regard himself succeeding in his plan (getting the inheritance), but the result (killing Alfreda) didn't occur.
Herring, Criminal Law: Text, Cases, and Materials (8 edn, 2018). p. 127
But what is the ordinary meaning of intention? The courts have not told us because (presumably) they think it is obvious. [1.] The widely accepted view is that the defendant ["D"] intends a consequence of his action if he acts with the aim or purpose of producing that consequence. Lord Asquith in Cunliffe v Goodman 12 explained that intention ‘connotes a state of affairs which the party intending . . . does more than merely contemplate: it connotes a state of affairs which, on the contrary, he decides, so far as in him lies, to bring about.’
It should be remembered that the jury will need to be persuaded beyond reasonable doubt that the defendant intended the result. In Haigh 13 there was clear evidence that the defendant had smothered her child. However, there was no evidence as to how or in what circumstances she had done this. In such a case the Court of Appeal held the jury could not have been persuaded beyond reasonable doubt that the mother had intentionally killed the child. She could therefore be guilty of manslaughter, but not murder. A useful test for seeing whether a result was the purpose of the defendant is to rely on Antony Duff’s test of failure:14 [2.] had the result not occurred would the defendant regard
himself as having failed in his plan? Consider the following case. David throws a burning rag into Veronica’s house, wanting to frighten her by causing a fire. The rag in fact sets fire to the house and Veronica is killed. Here, had the rag not caused a fire and so Veronica had not been frightened, David would have regarded his enterprise as a failure. David therefore intended to cause the fire. However, had Veronica lived, David would not have regarded the enterprise as a failure (he wanted to frighten her, not kill her) and therefore he did not intend to kill Veronica.15
Although Duff’s test is a very useful one there is one set of cases where it has to be treated with caution. That is where a result is a means to achieve a desired end. Imagine Martin kills his great aunt Alfreda in order to get his inheritance. The purpose of Martin’s action was to get the inheritance. Using Duff’s test of failure we could say that Martin would be delighted if Alfreda had lived, but he had somehow got hold of her money. However, it is generally agreed that Martin would be said to intend his great aunt Alfreda’s death. This is because under his plan the desired result (obtaining the inheritance) will be achieved through the means of killing her. So when we consider the purpose of the defendant this includes not only the aim, but also the means he wants to use to achieve that end.
To further clarify the core notion of intention, it is useful to distinguish it from other concepts.