Episode 11: It Must Be True

must: logical conclusion — Sherlock Holmes, 1890s (B1-B2)

Grammar Box

Meaning: “Must” for deduction expresses strong logical conclusions based on evidence—when you’re almost certain something is true because the facts point to it.

Form: must + base verb (present deduction); must be + adjective/noun

Example 1: “She’s not answering. She must be sleeping.” (logical conclusion from evidence)

Example 2: “You must be tired after working all night.” (deduction based on the situation)

Common mistake: Wrong: “It must is true.” Better: “It must be true.” (must + base verb, not must + is)

The Challenge

Luna heard footsteps upstairs in her supposedly empty apartment building. “Someone is up there,” she said nervously. Professor Wisdom appeared, raising an eyebrow thoughtfully. “Do you know for certain, or are you making a logical deduction based on what you hear?” Luna considered the difference between stating facts and reasoning from evidence. The watch glowed with a sharp, clear light, transporting them to a foggy London street where deductive reasoning had become an art form.

The Journey

Arthur Conan Doyle sat in his study in 1890s London, crafting a scene where his famous detective Sherlock Holmes would demonstrate the power of logical deduction. Though Holmes was fictional, Doyle based his reasoning methods on real scientific principles he had learned as a medical doctor. Holmes represented the Victorian faith in rational thought and scientific observation, believing that careful analysis of evidence could reveal hidden truths.

In the scene Doyle was writing, Holmes examined a walking stick left in his office. Without meeting the owner, he deduced the man’s profession, habits, and recent history. “He must be a country doctor,” Holmes tells Watson, pointing to specific scratches and wear patterns. “The stick shows signs of constant outdoor use, and the marks here must be from a dog’s teeth—he must walk frequently with a faithful companion.”

This method wasn’t mere guesswork but logical inference. Each “must” represented a conclusion drawn from observable facts. Doyle understood that readers found this reasoning compelling because it transformed observation into certainty through pure logic. Holmes never said “might be” or “could be” when the evidence was strong; he said “must be” to indicate logical inevitability.

The character’s confidence came from systematic thinking. When Watson saw random details, Holmes saw patterns that must lead to specific conclusions. A muddy boot must mean recent travel. Specific tobacco ash must indicate a particular brand. Multiple small facts, when combined logically, must point to one truth. This approach revolutionized detective fiction and influenced real criminal investigation methods.

The smell of tobacco and old books filled Doyle’s study as he refined Holmes’s dialogue. Outside, London fog pressed against the windows, creating the perfect atmosphere for mysteries that must be solved through reason rather than luck. Doyle smiled as he wrote Holmes’s famous line: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

The Deep Dive

“Must” for deduction expresses near-certainty based on logical reasoning from available evidence. When you say “He must be rich” after seeing his expensive car and mansion, you’re making a logical conclusion, not guessing. This differs from “must” for obligation (“You must wear a seatbelt”). Context determines which meaning applies, but deductive “must” typically appears in statements about situations, not commands.

The strength of “must” for deduction lies between knowledge and assumption. You don’t know with absolute certainty, but the evidence makes the conclusion almost inevitable. “She must be at home—her car is in the driveway and the lights are on” shows how multiple pieces of evidence support the deduction. However, you could still be wrong, which is why we use “must” rather than “is definitely.”

Don’t confuse logical deduction with weaker possibilities. “Must” indicates strong certainty (95% sure), while “might” or “could” indicate lower certainty (30-50% sure). Never use “must” for wild guesses without evidence. Also, the negative is “must not” or “mustn’t” for obligation, but “can’t” or “cannot” for logical deduction: “She can’t be home—I just saw her at the store” (not “she mustn’t be home”).

More Examples

History: “The pyramids must have required thousands of workers—the scale is too massive for small teams.” (logical conclusion)

Science: “This fossil must be millions of years old based on the rock layer we found it in.” (evidence-based deduction)

Everyday: “That must be your brother—you two look exactly alike!” (conclusion from observation)

Formal: “The results must indicate a systematic error, given the consistent pattern of deviation.” (professional deduction)

Informal: “You must be joking! That can’t possibly be true.” (strong disbelief based on logic)

Contrast: “You must wear safety gear” (obligation, rule) vs “You must be the new manager” (deduction from context)

Practice & Reflection

Exercises:

1. Fill in the blank: She’s wearing a wedding ring. She __ (must/might) be married.

2. Correct the mistake: “He must is very intelligent based on his achievements.”

3. Choose and explain: Which shows stronger certainty?
   a) “It might be expensive.”
   b) “It must be expensive.”

4. Rewrite: Change to show logical deduction: “I think she’s the manager because everyone asks her questions.”

5. Compare: Explain the difference: “You must arrive on time” vs “You must be tired.”

6. Your reflection: Think of a conclusion you’ve drawn from evidence recently. Write it using “must” for deduction.

Answer Key:
1. must (strong logical conclusion from visible evidence of ring)
2. He must be very intelligent (must + base verb “be,” not “must is”)
3. (b) — “must” shows near-certainty from evidence; “might” shows mere possibility
4. She must be the manager—everyone asks her questions (deduction from observed behavior)
5. First is obligation/rule; second is deduction from evidence (looks tired)
6. Check: Does your sentence use “must + base verb” based on observable evidence?

The Lesson

Back in her apartment, Luna listened again to the footsteps above. “Someone must be moving in upstairs,” she told Professor Wisdom with new confidence. “I hear furniture dragging, and the building manager mentioned a new tenant.” The Professor nodded approvingly. “Exactly. Sherlock Holmes taught us that ‘must’ transforms observations into logical certainty. You’re not guessing—you’re reasoning from evidence. That’s the power of deductive must: it shows your conclusion isn’t random but built on solid facts that point to one inevitable truth.”