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?