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The development of equity did not stop centuries ago. The innovation of major principles has occurred several times in recent history. One example was discussed in Chapter 4 on Case Law: the case of Central London Property Trust v High Trees Ltd (1947). It holds that a person who is already in a contract, and who has made a promise by which he intentionally modifies his contractual rights against another party, will not be allowed to resile from such a promise.

Source: P131, How the Law Works, Gary Slapper

  1. Would someone please explain the by? Why is there a preposition at all? Why not another preposition?

  2. Why not use the relative pronoun THAT instead:

...and who has made a promise by which THAT he intentionally modifies his contractual rights against another party...

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The "by" begins a separate (sub-)clause. It may be clearer if we split that off into its own sentence.

...who has made a promise. By that promise, he intentionally modifies...

(The bolded part is the replacement for the pronoun "which".)

Now, to your questions.

  1. In this case, "by" means "as a result of". This preposition is used because any other preposition would either change the meaning, or mean the same without necessarily being any clearer.
  2. Your proposed replacement changes the meaning. It now says he promises to modify his contractual rights, not that he makes some (unspecified) promise, and one effect of the promise is to change his rights. The difference is subtle, but probably important in a legal situation.

The change in meaning is lessened if we remove the word "he" as well as "by which":

...and who has made a promise that intentionally modifies his contractual rights...

This means the same thing as the original, and may be clearer. It might not be good style, though, because it seems to say that the promise intended the change. A sensible reader would know that is not what was meant, but I guess the writer wanted to say exactly what he meant.

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...who has made a promise.
By making the promise, he intentionally modifies his contractual rights ...

In most cases "that" would also work, but since "that" would link to the promise, not the person, one would have to use a passive construction. In that passive construction, you could not express that the person did it intentionally.

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