Habeas corpus, as far as I understand it, is simply a writ that a detained individual is being held to await the judgement of a legally constituted court of the validity of charges against him:
corpus…habeas…ad subjiciendum et recipiendum ea quae curia nostra de eo adtunc et ibidem ordinare contigerit in hac parte.
It is a constitutional principle only insofar as the writ demands that proof be presented to the court that the detention of the individual is lawful; it also states that without such convincing proof, the court will release the individual. A detained individual has, therefore, the right to challenge the charge and evidence against him before he is formally tried. If the charge and evidence are found to be valid by the court, he will then be remanded to await trial.
There appears to be some debate, at the Devil’s Kitchen and at Tom Paine’s, about whether this appears in Magna Carta. It doesn’t, obviously, as Magna Carta is a charter of liberties, not a legal writ that refers to the detention of a specific individual. However, Magna Carta does protect and confirm the legal necessity for writs of habeas corpus in Article 39:
nullus liber homo capiatur, vel imprisonetur, aut disseisiatur, aut utlagetur, aut exuletur, aut aliquo modo destruatur, nec super cum ibimus, nec super cum mittemus, nisi per legale judicium parium suorum vel per legem terre.
The necessity of providing proof that an accused individual ought to be detained, and his right to challenge that proof before a court, already existed in legem terrae; Magna Carta confirms them.
Tom Paine says: ‘For the first time in history, it limited the power of the state. It ended the rule of men and began the rule of law.’
This is not entirely accurate; what Magna Carta actually does is enumerate legal principles that already existed, but which John had routinely ignored or infringed; for the first time, Magna Carta enshrined what all men already held to be true, that the monarch was bound to observe his own laws. That Magna Carta had to be written, and John had to sign it, is merely a function of common law: it created a recorded precedent, thus overriding what had already been convention.
It is also slightly unfair to say that ‘you will be disappointed by’ the other articles.
No. 2 confirms the level of payment of relief upon inheritance – in other words, the Crown cannot demand extortionate inheritance taxes.
No. 3 confirms that underage heirs are not liable for relief/inheritance tax.
No. 4 confirms that trustees cannot plunder an underage person’s inheritance.
No. 7 confirms that widows do not have to pay relief/inheritance tax upon their husband’s death.
No. 9 confirms that only movable goods may be seized for payment of debt – not homes or land.
No. 12, 14, 15 and 16 confirm that no scutage (payment in lieu of service, i.e. tax) can be levied without the consent of those who would be paying it, and even then the Crown cannot demand more than what is reasonable and has been agreed upon.
No. 17, 18, and 19 confirm that people must be tried for crimes in the jurisdiction where they reside or in the jurisdiction where the crime took place.
No. 20 confirms that fines for offences cannot be levied arbitrarily, must be proportionate to the offence, and cannot result in the deprivation of livelihood.
No. 24 confirms that courts held by inappropriate authorities are invalid.
No. 27 confirms that if a man dies intestate, the Crown cannot seize his effects.
No. 28, 30, and 31 confirm that the Crown may not take a man’s property without payment.
No. 32 confirms that the Crown may not freeze or otherwise control a convicted felon’s assets for more than a year and a day.
No. 35 confirms standards in weights and measures.
No. 36 and 40 confirm that the Crown may not deny, delay, or sell justice.
No. 38 confirms that no man may be tried on the basis of hearsay or without the evidence of independent witnesses.
No. 42 confirms the free right of movement into and out of the country.
No. 45 confirms that only men who know and observe the law may be appointed to enforce or decide it.
Each one of these is tremendously important and not a function or product of circumstances limited to 1215. How many of them, I wonder, has the current government infringed?