Week 4 Session: Three conditions, one judgement

Judge the same fictional person's three accounts of one lock, choose the condition you trust, write down the rule you used, and examine what your own judging depended on.

15 March 2027

Retrieve your Week 3 record · 2 minutes#

Bring back your annotated reconstruction and retained judgement from Week 3. Keep the original unchanged. After reading Rowan’s three responses, write two sentences: which gap in your reconstruction would each condition reveal or conceal, and does that change your original judgement? If you have no record, use Priya’s worked reconstruction from Week 3, or write a short source-closed reconstruction from this week’s lock explanation before proceeding.

Purpose and preparation#

This self-guided investigation applies Week 4’s lesson. Allow about 25 minutes. Everything you need is on this page — you do not need the lecture open, a lock, any tools, or any paid or AI resource. Work alone or with a partner. Rowan and the three responses are fictional teaching examples written for this lesson, not transcripts and not research data.

You are practising a judgement, not grading a person. If you decide Rowan understands the lock, that is a result. The purpose is to find out what your judgement rested on, which is a different question from whether it was right.

If you have already read all three responses in the lecture or the deck, you cannot un-read them. Note that as a condition of your own judging and carry on; first impressions that have already formed are exactly the kind of thing this week is about.

1. The lock, in brief — 3 minutes#

A pin tumbler lock has a fixed outer casing, and inside it a cylinder that can rotate, called the plug. The key enters the keyway at the front of the plug. At the back of the plug a cam pulls the bolt out of the door frame when the plug turns.

Across the boundary between plug and casing sits a row of small stacks, usually five or six. Each stack is a key pin at the bottom, resting in the plug; a driver pin above it; and a spring above that, pressing the stack down. The shear line is that boundary itself — the join where the outside of the plug meets the casing. It is not a part; it is the line the plug must turn along.

With no key in, the springs push each stack down so the driver pins cross the shear line into the plug and hold it. A correctly cut key lifts each key pin by exactly the amount that puts the junction between that key pin and its driver pin level with the shear line. Nothing then crosses the boundary, so the plug turns and the bolt withdraws. A wrong key lifts the stacks by the wrong amounts: too little and a driver pin still hangs into the plug, too far and a key pin is pushed up into the casing. Either way something crosses the shear line. There is no partly open — one crossing anywhere is enough.

Technical grounding: Wikipedia, “Pin tumbler lock”. Keep this section visible for the whole activity; nothing here is a memory test.

2. What Rowan was asked, and what Rowan said — 5 minutes#

The same fictional person, the same lock, the same afternoon, three conditions. A condition is a feature of the situation an answer was produced in that could itself change the answer: what could be consulted, who the answer was for, and what kind of question was asked.

Condition (a) — the reference visible. Rowan has the text in section 1 open on the table, and is asked to explain how a pin tumbler lock works.

A pin tumbler lock has a plug that rotates inside a fixed casing. The key goes into the keyway at the front of the plug. Along the boundary between the two there are five or six stacks of pins, and each stack is a key pin at the bottom, sitting in the plug, a driver pin above it, and a spring above that pressing down.

That boundary is called the shear line. With no key in, the springs push the stacks down so the driver pins cross the shear line into the plug, and that is what stops it turning.

The key pins are different lengths, and the key is cut to match them. The correct key lifts each key pin so that the junction between the key pin and the driver pin ends up exactly at the shear line. Then nothing is crossing the boundary, the plug turns, and the cam at the back withdraws the bolt. A wrong key lifts the pins by the wrong amounts, so something still crosses the shear line and it stays locked.

Condition (b) — from memory, to a novice. The reference is removed. The listener has never wondered what happens inside a key.

OK, so — you know the zigzag notches along the top edge of a key? They are not decoration. They are a code, and the lock is the thing that reads it.

Inside the lock there is a barrel that has to spin round for the door to open. Normally it cannot, because there is a row of little pins, five or six of them, sticking down through the join between the barrel and the metal around it. A pin sitting across a join is a bolt. Nothing is turning.

Every pin is a different length and every notch is a different depth. The right key lifts each pin by exactly its own amount, so all of them come flush with that join at the same moment. Nothing is sticking across it any more, the barrel spins, and a little arm at the back pulls the bolt out of the frame.

A wrong key does lift the pins. It just lifts them to the wrong heights, so one is always left poking across the join, and it stays shut.

Condition (c) — a prediction about a changed case. Still nothing to consult. Rowan is told that on a key which works this lock, one single cut — the third one along — has been filed a little too shallow, and every other cut is correct. What happens when it is turned, and why?

Sorry — do you want me to guess, or is there a right answer? … Right. OK.

Too shallow means it does not lift that pin far enough. So that one is sitting low. The bit of it that is supposed to end up level with the join is below the join instead, so something is still crossing, at that one position.

And — this is the bit I am least sure about — I think it does not matter that the other five are fine. One is enough. Because the barrel is one piece. It is not five separate locks that each open a fifth of the way. So it does not half turn or feel loose. It just does not turn, the same as a completely wrong key.

Hold on. Would you even be able to tell this key from a totally wrong one, from the outside? I do not think you would. That feels wrong to me but I cannot see what would make it false.

3. Judge each response — 6 minutes#

Fill this in before reading the comparison. Preserve your original wording; if you change your mind later, write the second version underneath rather than replacing the first.

Then write one sentence answering the blunt question: does Rowan understand how this lock works?

4. Choose a condition and state your rule — 4 minutes#

Write down which single condition you would trust most, one sentence of reasoning, and one sentence giving the strongest objection to your own choice.

Then finish this sentence in one line, and keep it: “I counted that as stronger evidence because…”

5. Design a fourth condition — 4 minutes#

Invent one more condition for the same lock and the same person. Change one thing only: what may be consulted, who the answer is for, how much warning there is, or what kind of question is asked.

Write three lines: the condition, what evidence you expect it to produce that the first three did not, and the most likely way it could fail for a reason unconnected to the lock.

6. Examine your own judging — 3 minutes#

Keep this record. It is the thing you carry out of Week 4.

On consistency, be careful about what a clean result means. Three judgements pointing the same way show that you applied something stable. They do not show that the something was a good criterion, or that you could state it. The common version is forming a view under one condition and reading the other two through it without registering that a condition was chosen at all. If nothing would have changed your verdict, the consistency came from a commitment rather than from the evidence.

7. Working with a partner, and working alone#

With a partner. Both judge all three responses independently and in writing before comparing. Then find the first point at which your records diverge and try to state it as a disagreement about which condition counts, rather than about Rowan. Neither of you has to concede.

Alone. Do sections 3 to 6 as written, then leave the page and come back to it later in the day. Re-judge response (c) on its own, without rereading (a) and (b), and compare with your first record.

The solo route loses something real and it is worth naming: a second judgement by you is not an independent judgement. You already know your first verdict, and a partner’s disagreement is evidence you cannot manufacture for yourself. Your own account of why you judged as you did is self-report, not an observation of how the judgement was produced.

What this investigation does and does not establish#

You have produced a written judgement, a stated reason and a named objection, which is more than an intuition and can be inspected by someone else. You have not established that your rule is a good one, because nothing here tested it against anyone else’s.

That is the gap Week 5 addresses, by building an explicit instrument: a stated ladder of performances with each rung defined well enough that two people can apply it and locate their disagreement. Do not draft one yet — keep your one-line rule from section 4 exactly as you wrote it, and bring it. It is the raw material.

For the reasoning behind all of this, read the Week 4 lecture notes or work through the Week 4 slides.