This is a very important passage both capturing practice to the date of Blackstone’s writing and setting out a common law method which is still highly influential.

Of the common law, Blackstone wrote,

This unwritten, or common, law is properly distinguishable into three kinds:
(1) General customs; which are the universal rule of the whole kingdom, and form the common law, in its stricter and more usual signification….
As to…the common law, properly so called; this is that law, by which proceedings and determinations in the king’s ordinary courts of justice are guided and directed.

He gives specific instances, adding,

and an infinite number of minuter particulars, which diffuse themselves as extensively as the ordinary distribution of common justice requires.
Thus, for example, that there shall be four superior courts of record, the Chancery, the King’s Bench, the Common Pleas, and the Exchequer…—all these are doctrines that are not set down in any written statute or ordinance, but depend merely upon immemorial usage, that is, upon common law, for their support.

Some have divided the common law into two principal grounds or foundations: …Established customs…: and …Established rules and maxims…

But I take these to be one and the same thing. For the authority of these maxims rests entirely upon general reception and usage: and the only method of proving, that this or that maxim is a rule of the common law, is by showing that it has been always the custom to observe it.

But here a …question arises: how are these customs or maxims to be known, and by whom is their validity to be determined?

The answer is, by the judges in the several courts of justice. They are the depositaries of the laws; the living oracles, who must decide in all cases of doubt, and who are bound by an oath to decide according to the law of the land. The knowledge of that law is derived from experience and study; from the
“viginti annorum lucubrationes,” …
and from being long personally accustomed to the judicial decisions of their predecessors.

And indeed these judicial decisions are the principal and most authoritative evidence, that can be given, of the existence of such a custom as shall form a part of the common law. The judgment itself, and all the proceedings previous thereto, are carefully registered and preserved, under the name of records, in public repositories set apart for that particular purpose; and to them frequent recourse is had, when any critical question arises, in the determination of which former precedents may give light or assistance….

For it is an established rule to abide by former precedents, where the same points come again in litigation:

as well to keep the scale of justice even and steady, and not liable to waver with every new judge’s opinion;

as also because the law in that case being solemnly declared and determined, what before was uncertain, and perhaps indifferent, is now become a permanent rule, which it is not in the breast of any subsequent judge to alter or vary from according to his private sentiments:

he being sworn to determine, not according to his own private judgement, but according to the known laws and customs of the land;

not delegated to pronounce a new law, but to maintain and expound the old one.

Yet this rule admits of exception, where the former determination is most evidently contrary to reason; much more if it be clearly contrary to the divine law.

But even in such cases the subsequent judges do not pretend to make a new law, but to vindicate the old one from misrepresentation. For if it be found that the former decision is manifestly absurd or unjust, it is declared, not that such a sentence was bad law, but that it was not law; that is, that it is not the established custom of the realm, as has been erroneously determined.

And hence it is that our lawyers are with justice so copious in their encomiums on the reason of the common law; that they tell us, that the law is the perfection of reason, that it always intends to conform thereto, and that what is not reason is not law. Not that the particular reason of every rule in the law can at this distance of time be always precisely assigned; but it is sufficient that there be nothing in the rule flatly contradictory to reason, and then the law will presume it to be well founded.

And it has been an ancient observation in the laws of England, that whenever a standing rule of law of which the reason perhaps could not be remembered or discerned, has been wantonly broken in upon by statutes or new resolutions, the wisdom of the rule has in the end appeared from the inconveniences that have followed the innovation.

The doctrine of the law then is this: that precedents and rules must be followed, unless flatly absurd or unjust; for though their reason be not obvious at first view, yet we owe such a deference to former times as not to suppose that they acted wholly without consideration.

He gives an illustration and then says,

So that the law, and the opinion of the judge, are not always convertible terms, or one and the same thing; since it sometimes may happen that the judge may mistake the law.

Upon the whole, however, we may take it as a general rule,
“that the decisions of courts of justice are the evidence of what is common law:”
in the same manner as, in the civil law, what the emperor had once determined was to serve for a guide for the future.

The decisions therefore of courts are held in the highest regard, and are not only preserved as authentic records in the treasuries of the several courts, but are handed out to public view in the numerous volumes of reports which furnish the lawyer’s library.

These reports are histories of the several cases, with a short summary of the proceedings, which are preserved at large in the record; the arguments on both sides, and the reasons the court gave for its judgment; taken down in short notes by persons present at the determination. And these serve as indexes to, and also to explain the records, which always, in matters of consequence and nicety, the judges direct to be searched.
… Some of the most valuable of the ancient reports are those published by Lord Chief Justice Coke; a man of infinite learning in his profession, though not a little infected with the pedantry and quaintness of the times he lived in, which appear strongly in all his works. However, his writings are so highly esteemed, that they are generally cited without the author’s name…
And thus much for the first ground and chief corner-stone of the laws of England, which is general immemorial custom, or common law, from time to time declared in the decisions of the courts of justice; which decisions are preserved among our public records, explained in our reports, and digested for general use in the authoritative writings of the venerable sages of the law.

Edited for ease of reading.Spelling modernised and formatting changed.