Note: Montesquieu had a rather idealised view of the constitutional arrangements here but nevertheless his views have been very influential.
6. Of the Constitution of England. In every government there are three sorts of power: the legislative; the executive in respect to things dependent on the law of nations; and the executive in regard to matters that depend on the civil law.
…By the third, [the prince] punishes criminals, or determines the disputes that arise between individuals. The latter we shall call the judiciary power, and the other simply the executive power of the state.
Again, there is no liberty, if the judiciary power be not separated from the legislative and executive. Were it joined with the legislative, the life and liberty of the subject would be exposed to arbitrary control; for the judge would be then the legislator. Were it joined to the executive power, the judge might behave with violence and oppression.
There would be an end of everything, were the same man or the same body, whether of the nobles or of the people, to exercise those three powers, that of enacting laws, that of executing the public resolutions, and of trying the causes of individuals.
By this method the judicial power, so terrible to mankind, not being annexed to any particular state or profession, becomes, as it were, invisible. People have not then the judges continually present to their view; they fear the office, but not the magistrate.
…But though the tribunals ought not to be fixed, the judgments ought; and to such a degree as to be ever conformable to the letter of the law. Were they to be the private opinion of the judge, people would then live in society, without exactly knowing the nature of their obligations.
The judges ought likewise to be of the same rank as the accused, or, in other words, his peers; to the end that he may not imagine he is fallen into the hands of persons inclined to treat him with rigour.
If the legislature leaves the executive power in possession of a right to imprison those subjects who can give security for their good behaviour, there is an end of liberty; unless they are taken up, in order to answer without delay to a capital crime, in which case they are really free, being subject only to the power of the law.
…As in a country of liberty, every man who is supposed a free agent ought to be his own governor; the legislative power should reside in the whole body of the people. But since this is impossible in large states, and in small ones is subject to many inconveniences, it is fit the people should transact by their representatives what they cannot transact by themselves.
Of the three powers above mentioned, the judiciary is in some measure next to nothing: there remain, therefore, only two; and as these have need of a regulating power to moderate them, the part of the legislative body composed of the nobility is extremely proper for this purpose.
Though, in general, the judiciary power ought not to be united with any part of the legislative, yet this is liable to three exceptions, founded on the particular interest of the party accused.
It is possible that the law, which is clearsighted in one sense, and blind in another, might, in some cases, be too severe. But as we have already observed, the national judges are no more than the mouth that pronounces the words of the law, mere passive beings, incapable of moderating either its force or rigour. That part, therefore, of the legislative body, which we have just now observed to be a necessary tribunal on another occasion, is also a necessary tribunal in this; it belongs to its supreme authority to moderate the law in favour of the law itself, by mitigating the sentence.
It might also happen that a subject entrusted with the administration of public affairs may infringe the rights of the people, and be guilty of crimes which the ordinary magistrates either could not or would not punish. But, in general, the legislative power cannot try causes: and much less can it try this particular case, where it represents the party aggrieved, which is the people. It can only, therefore, impeach. But before what court shall it bring its impeachment? Must it go and demean itself before the ordinary tribunals, which are its inferiors, and, being composed, moreover, of men who are chosen from the people as well as itself, will naturally be swayed by the authority of so powerful an accuser? No: in order to preserve the dignity of the people, and the security of the subject, the legislative part which represents the people must bring in its charge before the legislative part which represents the nobility, who have neither the same interests nor the same passions.
It is not my business to examine whether the English actually enjoy this liberty or not. Sufficient it is for my purpose to observe that it is established by their laws; and I inquire no further.
[Edited for relevance and ease of reading.]