One route reads the nonoccurrence of the consequence as an indication that the condition is false. Another relies on an understood necessary connection between the proposed condition and the result.
«لَوْ كَانَ فِيهِمَا آلِهَةٌ إِلَّا اللَّهُ لَفَسَدَتَا»
Had there been gods besides Allah, both would have been ruined.
It would be careless to equate this argument with every ordinary causal statement in which an effect might have multiple causes. The validity depends on establishing the specific entailment at issue.