Factually, it must be proved that 'but for' the acts (or omissions) of the accused, the relevant consequence would not have occurred in the way that it did. In other words, if you eliminate the act of the defendant would the prohibited have occurred anyway?
R v White [1910] 2 KB 124 FACTS: W put poison in a drink intending to kill his mother. She was subsequently found dead. It was not clear on the evidence whether she had drunk any of the liquid from the glass. Medical evidence showed that she had died from heart failure, not from poisoning. HELD: W was therefore acquitted of murder, there being no causal link between the consequence and his act. However, he was guilty of attempted murder.