"A Grand Memory For Forgetting"

Context: David Balfour, a Scots lad who is shipped off to enforced labor in the American Colonies by a rascally uncle, is rescued by a Jacobite adventurer named Alan Breck, a Highlander. The two make their way ashore on the Scottish coast and fall into a series of adventures and narrow escapes. A man named Glenure is killed by an unknown murderer, and the blame falls on David Balfour and Alan Breck. The two, pursued by the sheriff and a detachment of British soldiers, escape by speed and cunning. When they are safe, David Balfour finds himself thinking that Alan Breck had a hand in the murder, as an act of revenge. The older man assures the boy that he had no part in the act, but Breck knows who the murderer is. However, like the good Highlander he is, he has used himself and the boy to draw the authorities off on a wild-goose chase. David has difficulty understanding the Highland mind and its workings, but his friend is persuasive:

". . . And do you know who did it?" I added. "Do you know that man in the black coat?"
"I have nae clear mind about his coat," said Alan, cunningly, "but it sticks in my head that it was blue."
"Blue or black, did ye know him?" said I.
"I could nae just conscientiously swear to him," says Alan. "He gaed very close by me, to be sure, but it's a strange thing that I should just have been tying my brogues."
"Can you swear that you don't know him, Alan?" I cried, half angered, half in a mind to laugh at his evasions.
"Not yet," says he; "but I've a grand memory for forgetting, David."