1. I don't understand the difference between these two examples, because in both cases, the goal is scientifically impossible!

  2. What does "if the means used to achieve the purpose are impossible" mean?

4.5 Impossibility and Common Law Offences

According to the House of Lords in DPP v Nock126 impossibility is a defence to a charge of common law conspiracy. However, this is only where the conspiracy could never be achieved: impossibility is not a defence if the means used to achieve the purpose are impossible [emphasis mine]. So if A and B conspire to project indecent images into the night-time sky, the fact that such a display was not scientifically possible would not prevent the possibility of a conviction for conspiracy to corrupt public morals. An example of where impossibility would be a defence to a common law conspiracy is where A and B intend to defraud V, while V had already died. It should be stressed that impossibility is assessed at the time of the agreement. The fact that the plan subsequently becomes impossible (e.g. the company it was planned to defraud has ceased to exist) does not prevent the original conspiracy being committed.

126[1978] AC 979 (HL).

Herring, Criminal Law: Text, Cases, and Materials (2020 9 ed). p 824.

2 Answers 2


I read it as that in the first case, their purpose (displaying indecent images to the public) couldn't be achieved by the means they had in mind (projecting into the night sky), but it could be achieved by other means (say, by having an airplane tow a banner around). So had they kept conspiring for longer, they might eventually have come up with some means to achieve their illegal purpose. Such a conspiracy is illegal.

On the other hand, defrauding the deceased V could never be achieved, no matter what means were used.


Why can you be convicted for conspiring to project indecent images into the night-time sky, but not conspiracy to defraud a dead person?

Short Answer:

One's a conspiracy to commit an offence and the other is an agreement to do something that isn't an offence.

Long Answer:

In Nock, the House of Lords ruled that one cannot unlawfully conspire, or attempt, to do an act when that act is not actually an offence in its own right - ie the impossibility defence.

The Nock defendants had conspired to produce cocaine, contrary to s.4 of the Misuse of Drugs Act 1971 by extracting it from another substance*.  However, there was no cocaine to be had, and they could not possibly produce it from that other substance. (I assume that they may have been swindled by an unscrupulous criminal associate in to the mistaken belief that they could do so.)

Similarly, there is no offence of defrauding a dead person as, to put it bluntly, they're dead; therefore any conspiracy to do so is also not an offence. (However conspiracy to defraud the deceased's estate or the heirs is, but that's not the example given and is off topic.)

  • For example, if someone told you that mixing daffodils and treacle makes cannabis, then any agreement with another person to try it would not be a criminal conspiracy as it's impossible to make cannabis this way.

Compare this to the example given of A and B conspiring to project indecent images, presumably contrary to s.2 of the Obscene Publications Act 1959. This is where the emboldened phrase comes in: Just because they do not have the technical capability to commit the offence, this is not enough to rely on the impossibility defence. 

  • For example, we conspire to go and burgle a house but our car won't start so we don't bother going. We are guilty of conspiracy to burgle, but not of the substantive burglary offence.

(*Disolving or suspending illegal drugs in an "innocent" liquid, such as wine or spirits, to be chemically extracted later is a tried and tested method for drug traffickers in an attempt to bypass Customs.)

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