Supposition: A supposition is a sentence which begins with the word 'suppose' or some synonym. For example, someone who is thinking about current experiments on genetically modified crops might say, 'suppose these experiments do risk dangerous contamination of other crops...?' Such a sentence does not commit the speaker to the view that these experiments do carry a dangerous risk; he or she is simply speculating about what would be the case IF this were so. Reasoning from such starting points is often called 'what if' reasoning. This kind of reasoning is very common in theoretical contexts. The police have to use it a good deal too, for example asking 'What if Smith really was in Amsterdam at the time of the murder...?'1
1. Alec Fisher, Critical Thinking: An Introduction (Cambridge: Cambridge University Press, 2001), p. 241.