Saturday, November 19, 2011

How Do You Know That? (II)

On Wednesday in How Do You Know That?, I suggested that a surprisingly large percentage of the myriad "facts" that comprise our points of view are no more than hearsay and lore. On the flip-side...

"It's the master cylinder."

"It's the master cylinder? What's a master cylinder? How do you know it's the master cylinder?"

"The brake pedal on your car is attached to a lever. The lever is attached to a rod. The rod is attached to a plunger inserted into a sealed cylinder of brake fluid. So the rod and plunger are a kind of piston. The cylinder full of brake fluid is attached to pipelines that connect to each of the brakes.

When you press the brake pedal, the plunger is pushed into the tube compressing the brake fluid. The change in pressure is distributed down each of the pipelines. At each brake, the increased pressure expands other pistons causing the brake that push the brake shoes against the brake drums or brake rotors."

"And you know that because?"

"Because I could never afford to take my car to a mechanic."


"So I learned to do things myself, like changing brake shoes and bleeding brake lines and replacing master cylinders. When friends found out that I could do that, I ended up helping them do the same. You might say that I've become intimate with various braking systems."

"OK, so you know something about these master thingamawhats. But how do you know that this guy's problem is the master cylinder. He barely said ten words about it."

"Well, he described significant variance in how far he has to push the the brake pedal before the brakes engage. He also said that the brake fluid level is good. So, it's the master cylinder."

"But you don't know that! You can't know that. You're just guessing."

"So what do you think it is?"

"What do I think it is?"

"Yeah. You heard the guy's description of the problem. What's your diagnosis?"

"Uhhh... I don't know. I don't have any experience fixing cars."

"So let's say you're this guy. You're concerned that your brakes seem to engage randomly. You can ask one of us for help. Who'd you ask?"

"Well, I guess I'd ask you."

"Even though I don't know the answer."

"Well, yeah, I guess."

"Why? Why would you ask me if I don't know?"

"Well, hmm... I guess because have you experience with this kind of thing and you seem to know what you're talking about."

"But what if I'm wrong?"

"Uh... I'd try something else?"

To be sure, our minds are cluttered with manufactured facts, random associations and events that never took place, things we "know" that have no basis in reality, stuff that wouldn't survive the scrutiny of, 'How do you know that?'

On the flip-side, there may be even more that we do know that we fail to see or acknowledge, not "facts", but conclusions derived through deduction and induction. Way back in February 2010, in For the Sake of Argument, we talked about the basics of logical thought, how brick by brick, we can build sound conclusions from little factoids.

If you're good at logical thought (reasoning) then are countless facts you know that you don't know you know, facts that you were never taught or learned, facts that don't clutter because the don't exist until you need them, facts that don't depend upon the experiences of others.

The cool thing is that reasoning is a skill that is well understood and easily learned. It's just not one that we spend much time teaching or learning. Not only is good reasoning easy to learn; so are the common pitfalls of bad reasoning (see Fallacy).

Unlike facts, reasoning is not learned through memorization and recitation, it's learned through practice. It's a skill that can be honed with others: challenging each other's logic, pointing out fallacies, dissecting arguments that don't quite ring true, building and strengthening.

If you're good at reasoning, you can derive all of western music theory just by listening to music; you can derive operations such as addition, subtraction, multiplication and division just by playing with numbers; you can learn all the rules of chess by watching others play; you can figure out grammars for languages you don't speak; you can discern the meanings of words you don't know.

If you're good at it, that is.

Happy Sunday,


  1. Tef: I recently read two fascinating books on the topic of how we are often wrong: “The Invisible Gorilla”, and “Why We Make Mistakes”. The inside jacket blurb on the second book actually says “this book is intended to make you less sure of yourself, and that’s a good thing”.

  2. Thanks Sree, I'll have to check out those books.

    A book in a similar vein is "The Drunkard's Walk: How Randomness Rules Our Lives". It's explains why intuition is generally wrong, specially when it comes to pattern recognition and determination of cause.

    Without any quantitative data to back it up, my sense is that we often put too much credence into beliefs we'd best question and not enough into beliefs we'd best trust. The conservative (and perhaps most sound) approach would be to question everything we believe, perhaps regularly.

    No doubt we'd find some beliefs to be poorly founded or completely unfounded. The foundations of well-founded beliefs may have changed. There may be no causal relationship between activities that "work" and the results we attribute to them.

    On the other hand, there's a practicality to going with your gut and trusting what you "know". Questioning everything can be expensive.


    What about an anual inventory of beliefs followed by a spring cleaning?

  3. Your assumptions are your windows on the world. Scrub them off every once in a while, or the light won't come in.

    - Alan Alda -


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