Reconstruction#

How to use this lesson#

Read at your own pace; no presenter is needed. Allow about 40 minutes. You need something to write with and nothing else: no lock, no tools, no paid book, no AI account.

This week asks you to read a short explanation and then stop looking at it. Three slides carry that explanation. Once you leave them, the lesson asks you not to return until it says so.

The people and the written answers here are fictional teaching examples.

Where Weeks 1 and 2 left us#

Week 1 established the examiner: the person deciding what a performance shows. It separated what you observed from what you inferred, put the conditions an answer was produced under into the record, and argued — with Clark and Chalmers — that using a reference does not automatically disqualify a performance. It also warned that taking a source away adds a demand on memory, so you can end up testing something other than what you intended.

Week 2 found that a fluent explanation can still conceal missing parts and causes.

This week asks something different#

We are not re-arguing whether to remove a source. Week 1 settled that removal has a cost and has to be chosen deliberately.

The question now is what the removal makes visible, and how an examiner reads what comes out. If the source is gone and something is missing from what a person produces, that absence is evidence. This week is about what kind of evidence it is.

Two things a reader can do#

Imagine Priya (a fictional reader) opening a short explanation of how a door lock works. It makes sense as she goes. Each sentence follows from the last, and by the end she would say she understands it.

Call that consumption: taking in an explanation while it is in front of you, following it, and finding it coherent.

Consumption is not a lesser activity. You cannot rebuild a structure you have never met, so consumption is where the material comes from.

Reconstruction, as this course uses it#

Now Priya puts the explanation away and writes the mechanism out with nothing in front of her.

Reconstruction here means rebuilding the structure of an explanation without the source in view: naming the parts, saying where each one sits relative to the others, and giving the causal steps that make the thing work.

It is not reproducing the wording. A reconstruction in completely different words, with the structure intact, is a success.

Recall and reconstruction are not the same thing#

A reconstruction usually needs recall to supply its pieces. The two are related; they are not interchangeable.

Why a lock, and why this one#

A reconstruction can only be diagnosed if the thing being reconstructed has nameable parts. Then a gap is a specific missing item, rather than a vague sense that an answer was thin.

A pin tumbler lock — the ordinary key lock on a front door — has eight nameable parts, one named boundary and three states. You need no prior knowledge of locks, and you will not learn to open one. We are examining an explanation, not operating a mechanism.

One warning about this example#

Rozenblit and Keil used a cylinder lock as a test item, and asked participants to explain step by step how one could pick one. As in Week 2, this was a diagnostic question, a probe for a specific gap; here it informed self-assessment: their interest was in what people discovered about their own knowledge, not in the technique.

This lesson does not cover picking a lock, and nothing here should be read as instruction in it.

Rozenblit & Keil (2002), Study 1, Phase 4. Their finding returns later in this lesson.

The reference · 1 of 3: the outer parts#

An outer casing has a cylindrical hole in it. Inside that hole sits a rotating cylinder called the plug. To unlock the door, the plug has to turn.

The front face of the plug carries a straight slot, the keyway, which the key enters. At the back of the plug is a cam: a projecting piece that, as the plug turns, draws back the bolt — the metal bar holding the door shut.

The reference · 2 of 3: the pins#

Five or six holes are drilled down through the plug. Each one holds a key pin, cut to its own length and rounded underneath so that a key can slide across it.

Directly above each key pin, in a shaft in the casing, sits a driver pin, with a spring above it pressing down. A key pin and the driver pin above it make up one stack.

The boundary where the plug meets the casing is the shear line: the surface along which the plug has to rotate.

The reference · 3 of 3: the three states#

No key. The springs push each stack down, so every driver pin is forced part-way out of the casing and into the plug. Pins cross the boundary, and the plug cannot turn.

Correct key. Its cut edge lifts each key pin by a different, exact amount, so every key-pin/driver-pin junction comes level with the shear line. Nothing crosses the boundary. The plug turns, and the cam draws the bolt back.

Wrong key. Its cuts lift the stacks by the wrong amounts. Lifted too little, a driver pin still crosses into the plug; lifted too far, a key pin is pushed up into the casing. One wrong stack is enough to hold the plug.

Close the reference now#

Those three slides are the whole reference. Read them as often as you like now, because when you advance the lesson asks you to treat them as gone.

The next slide asks you to rebuild that mechanism from nothing. If you look back while writing, you will produce a copy, and the step after it will have nothing left to examine.

Technical grounding: Wikipedia, “Pin tumbler lock,” linked at the end. It grounds the mechanism only, and is not evidence for any claim about learning.

The honest problem with that instruction#

You can scroll back. Nothing prevents it, and nobody is watching you. A source removal you administer to yourself is not something another person can check.

Follow the instruction anyway, for a practical reason rather than a moral one: your reconstruction is the raw material for the next step, and a copied version gives that step nothing to work on. You would lose the lesson, not fail a test.

And if you did look back, write that down. “Reconstructed with one look at the reference” is a usable record. An unrecorded look is what ruins it.

That unverifiability is part of the week#

Treat it as material to study, not a flaw to paper over. Examining yourself means holding both roles at once: you are the person producing the performance and the examiner judging it. You cannot independently confirm your own account of the conditions.

That does not make self-examination worthless. It makes the condition — self-administered, unwitnessed — part of the record, exactly as Week 1 required. State it, and then claim less than you would otherwise.

A fuller reconstruction: a written model answer#

“An outer casing holds a rotating plug. The key goes into the keyway at the front of the plug, and a cam at the back pulls the bolt. Six holes run down the plug, each holding a key pin cut to its own length. Above each key pin, in the casing, sits a driver pin with a spring pressing down.

Plug and casing meet at the shear line. With no key, the springs push the stacks down, driver pins cross into the plug, and it jams. A correct key raises every junction level with the shear line, so nothing crosses it, the plug turns, and the cam draws the bolt. A wrong key lifts a stack too little, leaving a driver pin in the plug, or too far, pushing a key pin up into the casing.”

This is a model we wrote, not a record of anybody’s work.

Annotating the fuller version#

It is strong. Every part appears, the stacking relation is right, and all three states are explained with the boundary doing the causal work.

It still loses something. C — the reference said the correct key lifts each pin by a different, exact amount, because the pins differ in length. This version says a correct key raises every junction level with the shear line, without saying that differing cut depths on the key are what achieve it.

One C against an otherwise complete structure is a small and specific finding. That specificity is what the scheme is for.

Priya’s reconstruction (fictional)#

“A door lock has a barrel that turns when you put the right key in. Inside there are pins of different heights. The key pushes the pins up to the right heights, and then the barrel can turn and the door opens. A wrong key does not push them to the right heights, so it will not turn.”

Read it before the annotation on the next slide. Notice first that nothing in it is false.

Annotating Priya’s reconstruction#

P — four parts are gone: the springs; the driver pins, since Priya has one set of pins where the reference had two; the cam; and the bolt.

R — nothing is placed. There is a barrel, and pins are “inside,” but there is no casing and nothing about driver pins sitting above key pins.

C — the largest loss. “Right heights, so it turns” is a rule, not a mechanism. The shear line is gone, so nothing explains why correct heights permit rotation.

X — the wrong key appears, but only as under-lifting. Over-lifting is absent.

How to read the annotated difference#

Count the letters, then weigh them. Priya’s four P items and her one C are not equivalent findings.

A missing part name is often a vocabulary gap: she may hold the structure and lack the phrase “driver pin.” A missing causal step is a different matter. Without the shear line there is no account of why the plug turns, so “right heights, so it turns” could have been produced by a reader who never took in the boundary at all.

A lost noun and a lost “because” are not the same loss.

Worked response: what a complete reconstruction establishes#

It establishes that at that moment, with the reference out of view, this reader produced a structure containing those parts, those relations, those causal steps and the wrong-key condition. That is a real performance, and it is considerably more than handing the reference’s own sentences back.

It is also more inspectable than a rating of confidence would be. There is a written artefact, and we can point at the part of it we are talking about.

Worked response: what it does not establish#

Not that the structure was built rather than memorised. A reader who learned the reference by heart yesterday hands in the same page.

Not that the reference was actually closed, because nobody witnessed it.

Not that the same thing would happen for another mechanism, or for one not studied minutes earlier.

Not that the reader can use the structure on a question the reference did not answer — say, what a lock with one broken spring would do.

So reconstruction is an instrument, not a virtue#

It is stronger evidence than a fluent read-through, for one stated reason: the source is not supplying the structure. That is the whole claim.

It does not make reconstruction morally better than reading, and it does not make consumption a bad habit. Priya could not have reconstructed anything she had never read. Consumption is the scaffolding; reconstruction is a measuring instrument built on top of it.

Anyone who tells you to stop consuming and start reconstructing has quietly changed the subject to self-improvement.

Evidence that removing a source exposes gaps#

Rozenblit and Keil asked sixteen graduate students to rate how well they understood a list of everyday devices, then to write a detailed, step-by-step causal explanation of four of them, then to rate themselves again. The ratings dropped: a repeated-measures ANOVA across their five rating points gave F(4, 56) = 16.195, p < .001.

Being asked to produce the explanation, rather than being handed one, is what moved the ratings.

Rozenblit & Keil (2002), Cognitive Science 26(5), 521–562, Study 1. An empirical finding from sixteen graduate students at one university; their Study 2 repeated the pattern with undergraduates.

What that study cannot do for us#

Their measurement is a self-rating: a number the participant gives for their own understanding. A drop in that number shows that participants revised their opinion of themselves. It is not independent evidence about what any of them could produce.

Our annotated reconstruction is a step better, because the evidence is a page we can inspect rather than a number. That improvement is our adaptation of their design, not their finding, and one reader with no control group replicates nothing.

Why we restore the source instead of trusting your sense of a gap#

Here is part of the reason the annotation step exists: people differ widely in noticing that they have not understood something.

Chi, studying students explaining physics examples aloud, reports that poor learners acknowledged a comprehension failure about 15% of the time while good learners did so about 46% of the time. Priya, mid-reconstruction, may simply not feel a gap where there is one.

Restoring the source removes the need to notice. The reference finds the gap for you.

Chi (1998 manuscript), “Self-explaining,” in the section on self-repair, reporting Chi et al. (1989). Her students had the text in front of them, which is a different condition from ours.

Keep a short record#

Write five lines and keep them with your annotated page:

1. Condition: “Self-administered and unwitnessed; reference closed,” or “one look back.”
2. Observation: “Without the reference, I produced…”
3. Counts: how many P, R, C and X.
4. Interpretation: “This supports the claim that I can…”
5. Limit: “It does not show that I can…”

A useful limit names the evidence you are missing. “Explain a mechanism I had not just read” is one.

Priya’s record, as an examiner would write it#

Condition: self-administered, unwitnessed, reference closed on her own account.

Observation: she named a rotating barrel and pins of differing heights, and stated that correct heights permit rotation.

Counts: P 4, R 2, C 1, X 1.

Interpretation: she retained the outcome of the mechanism and one of its components.

Limit: the single C is the finding that matters. Nothing here shows she holds the boundary that makes the outcome follow.

What to take from Week 3#

Reconstruction makes structure visible because the source is not there to supply it. Read the result by the type of loss rather than the volume: parts, relations, causal steps, conditions.

Then state what the result establishes under the conditions that produced it. The performance was real; the claim about it should be narrow.

Consumption remains necessary. Removal is a measuring choice with a cost, not a better way to live.

What Week 3 leaves unresolved#

One task cannot settle the question. Priya’s page could be thin because she does not hold the mechanism, or because she writes tersely, or because eight minutes was short, or because a written paragraph is not the form in which she thinks.

A single reconstruction cannot distinguish those. So we need to compare the same person’s performances across conditions instead of trusting one of them.

Week 4 asks: why does the same person look more or less knowledgeable under different conditions? It reuses this lock under two or three of them.

Read further, with a question each#

Neither reading is required to complete the activities above.

Sources and your next step#

Technical grounding for the lock only: Wikipedia, “Pin tumbler lock”. It names the boundary the shear point; “shear line,” used here, appears in the same article and means the same surface.

Priya, her reconstruction and the model answer are fictional teaching examples, not reported data.

For the full written activity and a record to keep, open the Week 3 investigation. The lecture notes give a continuous reading version.