How we cite our quotes: (chapter.paragraph)
Quote #1
"The situation is fraught with complications. We do not know whether the enchantments we ourselves have placed upon it, for example, making it Unplottable, will hold now that ownership has passed from Sirius's hands. It might be that Bellatrix will arrive on the doorstep at any moment. Naturally, we had to move out until such time as we have clarified the position." (3.57)
Throughout Harry Potter and the Half-Blood Prince, we really begin to understand that magic has its boundaries and that even the greatest wizard in the world, Dumbledore, can't do everything. For example, the magic that Dumbledore and the Order of the Phoenix placed on Sirius Black's now empty house might be trumped by a greater, ancient magic that comes from within the house itself.
Quote #2
"The magic I evoked fifteen years ago means that Harry has powerful protection while he can still call this house 'home.' However miserable he has been here, however unwelcome, however badly treated, you have at least, grudgingly allowed him his houseroom. This magic will cease to operate the moment that Harry turns seventeen; in other words, at the moment he becomes a man. I ask only this: that you allow Harry to return, once more, to this house, before his seventeenth birthday, which will ensure that the protection continues until that time." (3.104)
It is interesting that Dumbledore's powers can only protect Harry until he comes of age. Why do you think Dumbledore will not be able to protect Harry after this? How does magic in Harry Potter protect children?
Quote #3
"Lord Voldemort has finally realized the dangerous access to his thoughts and feelings you have been enjoying. It appears that he is now employing Occlumency against you." (4.24)
Occlumency is a kind of magic used to ensure that no one can read your mind or your thoughts. Harry has an ability to read Voldemort's mind and thoughts as a result of his encounter with the Dark Lord when he was just a baby.