Halliday & Matthiessen (1999: 119):
Finally, the distinction between expansion and projection is less determinate than we have suggested. The logico-semantic relation of condition, which is prototypically construed as a form of enhancement, could also be construed as a kind of projection; and this is also brought out in the grammar. Conditions specify a potential and actualisable but non-actual situation. This potential situation can also be set up through projection:
If the power supply fails, what's the best thing to do?Supposing the power supply fails, what's the best thing to do?Say the power supply fails, what's the best thing to do?
Words such as supposing and assuming are verbs of projection which have come to function as conjunctions in conditional figures; while other words such as imagine and say retain more of their projecting force. Sometimes even variants of the same word have come to differ a little in their location on this cline: for example, suppose and assume seem closer to projection than their corresponding participial variants. This is an uncertain region in which a figure hangs in the air, so to speak, suspended between the hypothetical material plane and the semiotic one.
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Cf Imagine if that were true and What if that were true.