Pensieve

The Pensieve is a example of Mrs. Rowling's lack of ability to fully think things through when it comes to time magic. She says in the description that it "recreates memories … taking every detail stored in the subconscious"¹ and making of that memory something that can be entered, re-experienced, and explored.

So why am I including it in time magic instead of memory magic? Because the examples she gives of its use include Harry following details that were beyond ear-shot and out of the line of sight from the person whose memory he had entered. I get that the subconscious mind might well have all sorts of details captured about events that I cannot consciously access. Things that you saw, but do not consciously remember. Things that you heard, but did not understand at the time. Perhaps even things that you heard but were to faint for you to understand might be captured and the sounds amplified by the magic of the pensieve.

Snape could not know that James had drawn an "L. E." on his OWL papers.² This one is a minuscule detail, but one that Snape literally couldn't have known. The book says that Harry stood behind Snape, read the exam title over Snape's shoulder, then turned before seeing his father.³ He then had to move "across two aisles and up a third"⁴ meaning that James was behind Snape. The "L. E." then was not an unconscious detail that Snape observed but did not note in the sense that I might have all sorts of things in line of sight but not actually be focusing on them much less paying attention to them. This detail was literally invisible to him.

But if I cannot see it, if it is out of my line of sight, then it is not in my memory conscious or unconscious. Thus for Harry to be able to see this "L. E." we need something more than Snape's memory. Hence time magic. If the pensieve is invoking an element of time magic, the conundrum is solved. Details can be plucked out of the time stream to fill in those that you actually don't recall. Things that you could not have seen.

However, if this is true, if details are being drawn out of time, how did Slughorn ever expect to pass a modified memory off when giving it to Dumbledore?⁵ I think the answer is that the memory does influence what the pensieve shows, that you have not in fact time travelled⁶ but rather that the pensieve starts with the memory as a basis and, for lack of a better way of putting it, fills in the details. Except that these details are actual history.

And that is why Slughorn's modified memory is so obvious. Because it is modified, it corresponds to no actual history, so when the pensieve seeks in time for details to fill in, there aren't any. Anything that he himself does not provide is simply not present.

However, I suspect that a truly skilled practitioner of the mind magics would not only know this, but would be able to work around it. This would explain why Dumbledore says that Slughorn's attempt is "crudely done."⁷ Instead of trying to replace the memory with totally false imagined scene, he or she might, for example, splice two memories. Or perhaps judiciously change a single word here or there to subtly influence the perception that the viewer has of the memory, without preventing the pensieve from finding enough in the memory to locate it in time. To carefully edit only where the memory is most conscious, most present, and thus where details from history are least needed. The pensieve then would be merely a non-interactive holodeck viewer of such a modified detail, with no need to link it in time, since the practitioner edited a segment from which no details are missing. This would not be a "crude" modification, but rather one that Dumbledore might term more elegant, but, per his comments to Harry probably poses more risks to the practitioner's actual ability to recollect the unmodified event.⁸ It is important to note that if this is either known to be true or speculated to be possible in world, then memories would not be considered admissible evidence in a responsible court.