The language of working it out
Deduction has its own phrases, and most of them exist to say how far a piece of evidence will go.
Eight of them. What is each one for?
Match each expression to the job it does.
Somebody has been watering plot 14
An allotment site on the edge of a city. Plot 14 was given up two years ago when its holder moved away. It should be waist-high in weeds. It is not: it is watered, weeded and, last August, harvested.
Four people, recorded separately.
A · Yusuf, site secretary
It can’t be somebody from outside. The gate is a combination lock and the number changed in March, so whoever it is must have been given the new one — which means they are on my list of forty-one people, and I have known most of them for a decade.
What doesn’t fit is the water. The tank is metered. If somebody were filling cans from it twice a week for two years, the reading would show it, and it doesn’t. So either they are bringing water in, which is a lot of carrying, or I am wrong about the meter.
B · Kath, plot 12
I’ve been here since six in the morning most Saturdays for eleven years and I have never seen anyone on 14. That doesn’t mean nobody comes. It means they don’t come on Saturday mornings, which, if you think about it, is the one thing everybody here knows about me.
It might be Trevor. He had 14 before the Hendersons and he still walks the path on Sundays with the dog. But he’s eighty-one and those beans were staked properly, so I don’t know.
C · Priya, plot 27
Everyone assumes it must be one of us, and it might not be. There’s a gap in the fence at the bottom that has been there since the storm, and the council was supposed to fix it in November.
What I keep coming back to is the harvesting. Somebody took those beans and courgettes home and ate them. That’s not somebody keeping a plot tidy out of guilt. That’s somebody who needs the food, or somebody who is enjoying themselves enormously, and those two people would behave completely differently about the weeding.
D · Martin, plot 3
Nine times out of ten it’s the person who reported it. I’m not saying it’s Yusuf. I’m saying that’s usually how it goes.
Look, it doesn’t follow that whoever waters it took the beans. Two people can use one plot. And the fence gap doesn’t rule anything out either way, because a member could use the gap perfectly well — it’s closer to the car park than the gate is. Everyone here keeps solving this with the piece of evidence they happen to like.
Seven statements. Decide which speaker each is true of. A speaker may be the answer more than once.
A Yusuf
B Kath
C Priya
D Martin
1. Points out that their own absence of evidence proves only something about themselves.
2. Rejects a conclusion while accepting the fact it was drawn from.
3. Names the one detail their own theory cannot account for.
4. Argues that the same evidence supports two very different people.
5. Reasons from what usually happens rather than from this case.
6. Uses a locked gate to narrow the field to a named group.
7. Raises a possibility that everybody else has ruled out without testing it.
Strong and weak, and what stress does
Modals have two pronunciations, and the difference carries meaning.
Unstressed, must reduces: they must be on the list is /ðeɪ məs bi ɒn ðə lɪst/ — the /t/ usually disappears before a consonant. That is the ordinary, neutral deduction.
Stressed, it argues: they MUST be on the list says I have thought about this and there is no alternative — often against somebody who has just suggested one. Same word, same grammar, a different move in the conversation.
Read Yusuf’s first paragraph twice: once neutral, once with the modals stressed. The second reading is a man being contradicted.
Four sentences have been removed from a fifth account. Put each one back where it belongs.
I looked at the meter myself last Tuesday and Yusuf is right about the reading. [1] That is a long way to carry water twice a week for two years, and nobody does that for a plot which is not theirs.
Which leaves the rainwater butts. [2] It was empty in July, which is exactly when the courgettes would have needed the most.
[3] She is on the path at six every Saturday and she has never seen anybody, so whoever it is comes at a time when the site is empty — early Sunday, or late on a weekday evening.
None of which tells us who. [4] It tells us they know the site well enough to avoid the two people who are always here, and that is not a stranger through a fence.
Grading the claims
One or two words per gap. Each gap takes the strongest word the evidence in that sentence will support.
The gate combination changed in March, so whoever it is have the new number. The meter reading has not moved, so they be using the tank. Kath has never seen anybody on a Saturday morning, so they come at another time — although she simply have missed them. Trevor be involved, but at eighty-one he have staked those beans himself.
Six sentences from a discussion of plot 14. One error each — some grammar, some strength.
1. It mustn’t be somebody from outside — the gate is locked.
2. Whoever it is must to know the site well.
3. It must be Trevor, because he walks past on Sundays. — walking past is thin
4. Kath has never seen anyone, so there might not be anybody. — somebody is clearly watering it
5. The meter has not moved, so they might not be using the tank. — the meter settles it
6. He can be on the list — Yusuf gave him the number in March.
Four explanations, ranked and defended
Task card · Speaking · Groups of three · 12 minutes
You have the four accounts. Produce a ranked list of explanations for plot 14 — best first — and tag each one with its strength.
Four explanations, each with a modal you can defend, and one sentence of evidence for each.
- Every explanation gets a tag: must, might / could, or can’t.
- For each tag, say the evidence out loud. If nobody in the group can name it, the tag drops a level.
- Name the one detail your best explanation still does not cover. Every theory has one.
- Finish with the question you would ask next — and who you would ask it of.
The rule of the room. Anybody may challenge a must by asking "what rules out the alternatives?" If the answer is "nothing, it just seems likely", the claim becomes might. No argument, no vote.
Which claims survived
Two questions.
1. How many of your group’s must claims survived the challenge?
2. What did the surviving must claims have in common?
Evidence, then conclusion
Two questions. Twenty seconds.
1. What does a stressed MUST add?
2. What is the test for a must claim?
Next. The full present system — must, can’t, might, could, may — and exactly where each one sits.