The Meaning Of An Idea vs The Meaning Of A Fact

Halliday & Matthiessen (2014: 543):

 

The meaning of (a) in the preceding paragraph is that Mark Antony’s thinking brought the idea that Caesar was dead into existence (as with Mark Anthony believed/imagined that Caesar was dead); but the meaning of (b) is that the already existing fact that Caesar was dead impinges on Mark Antony’s consciousness (as with that Caesar was dead scared Mark Anthony).