Causation
Factual causation
The claimant must show that, but for the defendant's breach, the damage would not have occurred. This 'but for' test is the starting point. Where multiple possible causes exist and the balance of probabilities cannot single one out, the claim generally fails, unless a recognised exception applies (for example, where two or more defendants independently caused indivisible harm, or where a defendant materially increased the risk of harm). Questions often test whether the claimant has proved causation on the balance of probabilities.
Intervening acts and multiple causes
A later event may break the chain of causation if it is independent, unforeseeable and of such a kind that it renders the original breach merely part of the background. A claimant's own unreasonable act can break the chain; a foreseeable reaction or a reasonable attempt to mitigate usually does not. Where two causes operate concurrently and either alone would have caused the loss, the defendant may still be liable. Where a later cause would have produced the same loss anyway, the original defendant may escape liability for that loss.
Legal causation
Even if factual causation is established, the court asks whether the defendant's breach was a substantial cause of the damage. The damage must fall within the scope of the duty. Questions distinguish factual from legal causation: proving 'but for' is not enough if the harm is outside the risk created.
Worked rule of thumb
Map every possible cause and ask: would the loss have occurred anyway? If yes, no factual causation. If the loss would have occurred on a different route at the same time, the defendant may still be liable for hastening it. If a later independent act is extraordinary and unforeseeable, the chain breaks.
Traps
Do not accept 'the defendant's act was a cause' as sufficient; it must be a cause of the specific damage claimed. Watch for cases where the claimant cannot prove which of several defendants caused the harm, and for loss of chance arguments that courts treat cautiously.