A motif is a recurring theme, symbol or image in the text. Any work of literature will contain a number of different motifs. One in this great story is the dream that the story opens with, which we only learn to be based on reality at the end of the story when David finally reaches Sealand with Rosalind and Petra. What makes this dream so strange is that he is able to see things so vividly that he has never actually seen before in his life:
But this city, clustered on the curve of a big blue bay, would come into my mind. I could see the streets, and the buildings that lined them, the waterfront, even boats in the harbour; yet, waking, I had never seen the sea, or a boat...
The significance of this motif therefore lies in the way that it points towards the telepathic gift that David has and how it enlarges his mind to be able to see and experience things that he himself has never seen and experienced. The importance of this motif is cemented in the final section of the story, where David actually reaches this city and says "It was just as I had seen it in my dreams."