We examine how opinion on a controversial real-world issue shifts as a function of reading relevant arguments and engaging in a specific mental simulation about a future, fictional state of affairs involving the target issue. Individuals thought either counterfactually about a future event (“if only X had not happened …”) or semifactually about it (“even if X had not happened …”). In Experiment 1, as expected, individuals became more in favour of a course of action (the electronic tagging of children) (...) after reading relevant supporting arguments. They also became more in favour after they thought counterfactually how a negative future event could have been avoided if only electronic tagging had not been banned some years earlier. The effects of argument relevance and type of mental simulation were additive. Using a similar procedure but a different target issue (whether the UK should join the Economic Monetary Union), Experiment 2 confirmed that the type of mental simulation exerts a substantial effect on the impact of relevant arguments. Much weaker effects arose when the mental simulation was not relevant to the target issue. We set and discuss our results in terms of dual-processing theory. (shrink)