Mary's question jogged Russell's memory, and he cleared his throat.
"Yesterday, Charlotte and I stumbled upon an interesting case."
"An interesting case?" the girl raised her delicate brows, leaning in a little. "What sort of case would attract Charlotte's interest?"
"Well... to be brief, a woman named Holly David thinks she's being haunted by a ghost," Russell explained.
He described Holly David's troubles in detail, and Mary listened silently, blue eyes occasionally shining with thought or confusion.
"Hm... an invisible ghost, secretly collecting her used coffee cups, spraying her perfume, reading the books she's read, sleeping in her bed..."
Mary frowned slightly, thinking for a while. "There are two possibilities. One is that Holly David suffers from a mental illness, maybe dissociative identity disorder. Or... she's being stalked by a complete psychopath."
"So, from your point of view, which is more likely?" Russell asked.
"The latter, I think. If it's dissociative identity disorder, some things don't add up logically," Mary replied, making notes. "For example, how could the bed be warm? That's an obvious loophole."
But if it's really the latter, then the situation is even worse.
"If I had to choose, I'd rather deal with a dissociative personality than a psychopath. At least with the former, you're your own ally, and you won't get hurt."
Russell nodded in deep agreement.
"And so, what's the result?" Mary pressed. "Did you find the 'ghost'?"
"Unfortunately, no," Russell shrugged. "Miss David is staying with a friend for now."
He paused before continuing, "Charlotte and I searched her apartment thoroughly. Doors and windows were locked, no secret passages, no ventilation ducts. By every mark, whoever it was... appeared to materialize out of nowhere."
"They just showed up, huh..." Mary murmured thoughtfully, skimming a blank page on her notebook.
Then she picked up her pen, casually sketching a square and then a smaller square next to it.
Russell glanced over. It was a rough floor plan of the apartment.
"Does her place look about like this?" she asked, confirming.
"Yes, more or less," Russell nodded.
And Mary started to analyze.
"With doors, windows, and secret passages ruled out..." Her voice was soft, almost as if muttering to herself, or perhaps inadvertently posing a question to Russell. "Then what about the walls?"
She hesitated, idly pausing the pen tip on a black line.
"Like, breaking through them?"
"Impossible," Russell instantly denied. "All the walls are solid brick and stone. Trying to damage or break through them would make too much noise—there's no way you wouldn't be caught."
The neighborhood is mostly home to old folks who value peace and quiet.
Mary nodded, marking a tiny 'X' over the line for the wall.
"Honestly, listening to your explanation, my first guess was that maybe the ghost lives in Miss David's home," Mary said.
"Honestly, I don't see how that's possible," Russell shook his head. "It's a one-bedroom apartment—no place to hide anyone."
"So, this is just a hypothesis," Mary said, tracing the floor plan's lines lightly. "After all, isn't deduction a process of bold guesses and careful verification? You present a possibility, then systematically try to refute it using what's known."
"Sounds familiar... Charlotte did something similar," Russell mused. Charlotte had proposed the dissociative personality theory, but the logic didn't stand up on closer investigation.
In fact, these two were surprisingly alike in some ways.
"Let's try it this way—this time I'll set forth a hypothesis, and you try to refute it," Mary said softly. "I think this 'ghost' never actually left Holly David's apartment to begin with."
"Your turn to disprove it."
"Mm... as I said, the place is much smaller than imagined, not a lot of places a person could hide in a one-bedroom apartment."
"Frankly, even Moriarty couldn't pull something like this off."
"Logically, I agree," Mary nodded, though she didn't immediately argue. "But what if by 'hiding,' I meant something dynamic instead of static?"
"Dynamic?" Russell cocked his head, not grasping her intent.
"In hide and seek, the goal is simply not to be found; there's nothing that says you have to hide in just one spot," she smiled, flipping her notebook over so Russell could see better. "If there was a piece of renovated furniture in the apartment—say, a wardrobe with a movable back panel, or a very heavy sofa that's actually hollow—then when Miss David is home, it's just ordinary furniture, but once she leaves, the hidden person can move about."
"No way," Russell shook his head. "If it was that obvious, Charlotte would've noticed. Even if she didn't, I would've noticed."
Of course, he didn't say that last bit out loud.
"Plus, it would be difficult to modify furniture so carefully without leaving a trace."
"Mm... That's reasonable," Mary agreed, underlining "modified furniture" in her notes. She didn't seem frustrated at all—if anything, this only made her more interested.
Just as she was about to propose a second hypothesis, suddenly, the old professor's voice boomed from the podium.
The two had tried to keep their voices and movements down, but sitting in the back made it all too audible.
"Back row, Mr. Watson," the professor adjusted his glasses, scrutinizing Russell with displeasure. "Is my class that boring?"
[Professor Fields is quite dissatisfied with your laziness. Malice level +10.]
No sooner had he finished than the whole class fixed their gaze on Russell—part gloating, part annoyed.
"If you don't want to study, don't drag others down with you."
"Uh..." Russell slowly stood and scanned the blackboard, searching for a reply.
At that moment, he felt someone nudge him in the waist. Looking down, he saw Mary quietly open her notebook and point at the illustration.
Russell's eyes lit up.
"No, Professor Fields, your class is very interesting," he immediately said. "Mary and I were just discussing a structural mechanics problem."
The professor narrowed his eyes in skepticism, then slowly turned to Mary.
"Is that so, Miss Morstan?"
"Yes, Professor." Mary nodded politely as she stood. "Earlier while you talked about stress transfer and structural applications in closed systems, I had some doubts, so I was discussing with Russell. Sorry for interrupting the class."
[Thanks to Mary Morstan's support, a wave of envy rippled through the student body—your "malice level" increases by 30.]
No, still can't laugh...
Russell struggled to restrain his trembling lips, forcing a shrug at the professor.
Professor Fields seemed slightly taken aback, and after a pause, softened his tone.
"Having questions is good," he said. "But save discussion for after class, if you can."
After a moment, he continued, "So—what was your question?"