How to analyse: only if ‘but for'?

Question

Source: p 20, Criminal Law: The Basics, 1 ed (2009), by Herring

  As we have just noticed, most crimes involve proof that the defendant caused a harm. ... If the defendant stabs the victim, and the victim falls down dead, there can be little doubt that the defendant caused the death of the victim. However, there can be cases where the causation question is far from straightforward.
  A good starting point is the principle that the defendant can be said to have caused a result only if ‘but for’ his or her act the harm would not have happened. This is sometimes known as factual causation. So in one case (White) a defendant poisoned his elderly mother’s tea. Before she took a sip she suffered a heart attack. The medical evidence showed that her heart attack was unrelated to the poisoning. In other words, she would have died in exactly the same way and at exactly the same time had she not drunk the poison. The result was that he could not be said to have caused her death. However, he could be charged with attempted murder.

Based on the link, I know that but for = 1. except for. Still, even after substituting with this, I don't understand how to parse the bolded: only if 'except for'? Please explain and show all steps and thought processes; I’d like to try to resolve this myself in the future?

Answer 1

That's why good style encourages using positive instead of double negative clauses... and the quotes only confuse the matters more by separating the expression from the sentence it applies to.

‘but for’ his or her act the harm would not have happened.

this could be rephrased, by removing the double negative, as

only due to his or her act the harm would have happened.

And in this context the becomes quite clear.

the defendant can be said to have caused a result only if [only due to his or her act the harm would have happened.]

I guess the writer tried to avoid the repetition of 'only', and as result made this quite hard to comprehend.

"A is only true if only X is true." so, there are factors X,Y,Z and for the conclusion A to be true, X must be true, while Y and Z must be false. In all other cases A is false.

got transliterated as

*"A is only true if [if not for X being true, all factors would be false]"

...and phrased it with awkward punctuation and rare and unintuitive use of but.

A stylistically better and clearer phrasing would switch the phrases around a bit and add both variants of outcome:

the defendant can be said to have caused a result only if the harm would have happened due to his or her act, and not for any other reasons.