Recent tweets of my New Statesman colleague Laurie Penny  have attracted considerable attention. In particular, her apparent suggestion that what would normally be "criminal damage" is not actually violent and is permissible in the context of certain protests has been widely discussed, including this analysis  by CharonQC, the doyen of English legal blogging.
Of all bloggers, Laurie Penny is able to speak up for herself, and I do not propose to engage here with the details of that debate. However, there is a wider issue which warrants attention, and it is an issue on which every thoughtful and liberal person should have a view.
When is a criminal act permissible on political grounds? By criminal acts, I do not mean the simple and principled non-compliance which can be labelled "civil disobedience". I mean instead positive actions which breach criminal law, such as offences against the person or against property. It is probably clear what sort of offences go beyond simple civil disobedience, though there may be grey areas at the margins.
Many individuals seeking or exercising power would like to be excused from criminal liability on political grounds, from the terrorist and the dishonest politician, to the troublemaker throwing paint at Topshop and the racist thug. It would seem that for each of these individuals, the criminal law is just for other people. Their self-serving sense of legitimacy checks and overrides the legitimacy of the state.
However, a political excuse cannot be enough to exclude criminal activity. Profession of a cause, like ignorance of the law, cannot always be a valid excuse. If it were, then everyone subject to the criminal law would invoke it. There must be a sensible limit to which politics can be used as a defence to a criminal charge.
On the other hand, very few would maintain that there can never be, in any circumstance whatsoever, a good political reason to commit what would otherwise be a criminal act. There may be differing views on when such an action would be justified, and many would only concede that it could happen only under imaginary and extremely illiberal laws, rather than those in force at the current time. For example, helping a member of a persecuted group to escape capture and execution could feasibly be a positive criminal act, and one would hope that many would selflessly commit the crime to assist a stranger.
There may be no perfect theoretical answer to this problem. In practice, the decisions of those in the criminal justice system, from the arresting officer to the sitting jury, should be informed by common sense and proportion. Politics may not be a defence, but there should always be a public interest in pursuing a prosecution. At the extreme, and with serious offences, it should always be open to a jury to find a defendant not guilty, regardless of the dismay and frustration of the coercive powers of the State.
Such a practical approach is haphazard. It certainly does not appeal to the tidy-minded. It offers no satisfying conceptual basis to those who want certain criminal acts to not have legal consequences. But what is the alternative? Should political commitment ever be defence in criminal law? And if so, how would it actually work?
David Allen Green is legal correspondent of the New Statesman and a practising media lawyer.