This thesis presents a series of experiments where endorsement rates, latencies and measures of cognitive ability were collected, to investigate the extent to which people search for counterexamples under necessity instructions, and alternative models under possibility instructions. The research was motivated by a syllogistic reasoning study carried out by Evans, Handley, Harper, and Johnson-Laird (1999), and predictions were derived from mental model theory (Johnson-Laird, 1983; Johnson-Laird & Byrne, 1991). With regard to the endorsement rate data: Experiment 1 failed to find evidence that a search for counterexamples or alternative models took place. In contrast experiment 2 (transitive inference) found some evidence to support the search for alternative models under possibility instructions, and following an improved training session, experiment 3 produced strong evidence to suggest that people searched for other models; which was mediated by cognitive ability. There was also strong evidence from experiments 4, 5 and 6 (abstract and everyday conditionals) to support the search for counterexamples and alternative models. Furthermore it was also found that people were more likely to find alternative causes when there were many that could be retrieved from their everyday knowledge, and that people carried out a search for counterexamples with many alternative causes under necessity instructions, and across few and many causal groups under possibility instructions. .The evidence from the latency data was limited and inconsistent, although people with higher cognitive ability were generally quicker in completing the tasks.