Logic puzzles!

[rochvelleth]

I saw this on saraphale's journal and couldn't resist taking the test:

http://www.think-logically.co.uk/lt.htm

I got 15 out of 15, which leads me to mention a point of controversy raised...

Comments

[ext_72852]

I got that "wrong", having decided that to "predict" something didn't necessarily mean guaranteeing 100% accuracy. I figured we "can predict" when we're fairly sure...

[rochvelleth]

Yes, indeed. Of course, one can 'predict' in several different contexts, some of which imply more certainty than others, and that doesn't help either!

[weebleflip]

Likewise here.

[cartesiandaemon]

The question is whether we can predict that every future examination of something will reveal that said something has the same composition as that revealed by all past and present examinations of it.

Yeah. That's something like "scientific induction", which works in real life but is false in logic. I should probably have guessed what answer they wanted. But the more I think about it, the more I agree with saraphale that the other one is the right answer.

Unless they mean "water is by definition H2O, but there is a small chance that when you observe it in future, you will observe something different to what it actually is", but that's an excessively roundabout way of asking that.

Or maybe "water is now H2O, but maybe that changes over time".

Or conceivably, we accept as assumption that in the past, you can look at the chemical composition of molecules with a microscope, but have no such (false) assumption given about the future?

[rochvelleth]

Unless they mean "water is by definition H2O, but there is a small chance that when you observe it in future, you will observe something different to what it actually is", but that's an excessively roundabout way of asking that.

Or maybe "water is now H2O, but maybe that changes over time".

Yes, I assumed that was the sort of thing they'd cite when revealing the answer, but they didn't. *shrug* I thought the 'wrong' answer was more right, but I thought they were expecting the 'right' answer so gave it.

[naath]

I got the horse one wrong from failing to read...

I think that the water one is possibly incorrect because I think that we *can* predict that blahblahblah we just can not do so with absolute certainty! I can predict that from now until the end of time all cheese will be bright pink... I'd be wrong, but I can do it anyway. If it said "we can certainly know that all future examinations of water will..." then I think it'd be without question incorrect from a logical standpoint. Although it does raise questions about how one *defines* water.

[pw201]

Got them all right, as I thought question 15 was angling for Hume's ideas.

It's not a great example, though, because if we take the first premise as a definition, future examinations that don't reveal the same composition mean that your examination was wrong or you weren't looking at water. You might decide that the hypothetical substance discovered later should also be called water, but then you're invalidating the premise, which isn't what you're asked to do in the other questions.

Q15

[ewx]

Given the vagueness in defining "water" it might not be unreasonable to expand that vagueness to "Hydrogen" and cite Deuterium l-)