Of Interpretation, Blackstone wrote,

The fairest and most rational method to interpret the will of the legislator is by exploring his intentions at the time when the law was made, by signs the most natural and probable.

And these signs are either
the words,
the context,
the subject matter,
the effects and consequence, or
the spirit and reason of the law.
Let us take a short view of …       …
…the most universal and effectual way of discovering the true meaning of a law, when the words are dubious, is by considering the reason and spirit of it; or the cause which moved the legislator to enact it. For when this reason ceases, the law itself ought likewise to cease with it.

An instance of this is given in a case put by Cicero…There was a law, that those who in a storm forsook the ship should forfeit all property therein; and that the ship and lading should belong entirely to those who stayed in it. In a dangerous tempest all the mariners forsook the ship, except only one sick passenger, who, by reason of his disease, was unable to get out and escape. By chance the ship came safe to port. The sick man kept possession, and claimed the benefit of the law. Now here all the learned agree, that the sick man is not within the reason of the law; for the reason of making it was, to give encouragement to such as should venture their lives to save the vessel; but this is a merit which he could never pretend to, who neither stayed in the ship upon that account, nor contributed anything to its preservation.

From this method of interpreting laws by the reason of them, arises what we call equity,

For, since in laws all cases cannot be foreseen or expressed, it is necessary that, when the general decrees of the law come to be applied to particular cases, there should be somewhere a power vested of defining those circumstances, which (had they been foreseen) the legislator himself would have expressed…

Equity thus depending, essentially, upon the particular circumstances of each individual case, there can be no established rules and fixed precepts of equity laid down, without destroying its very essence, and reducing it to a positive law.
…on the other hand, the liberty of considering all cases in an equitable light must not be indulged too far, lest thereby we destroy all law, and leave the decision of every question entirely in the breast of the judge.

And law, without equity, though hard and disagreeable, is much more desirable for the public good than equity without law; which would make every judge a legislator, and introduce most infinite confusion; as there would then be almost as many different rules of action laid down in our courts, as there are differences of capacity and sentiment in the human mind.

Edited for ease of reading. Formatting altered.