[BoppreH]

I think it's also appropriate to use it when the rule is so strong that exceptions are famous because they are exceptions. "Birds are capable of flight" is strong enough that penguins and ostriches are famous for being counterexamples.

[SketchySeaBeast]

But that's not following the saying - it's still not proving, it's modifying the rule. It shifts the rule from "birds can fly" to "most birds can fly". Pointing out that penguins can't fly doesn't make the case that birds can fly stronger in any way.

[BoppreH]

You're right in a strict sense. But in my experience such strictness is only useful in hard sciences and (maybe) legalese. There are exceedingly few things we can claim to apply everywhere, and even fewer we can "prove" to each other.

Give it a try if you don't believe me. Even categories we take for granted, like trees and fish, are not perfectly crisp, and "obvious" facts like "humans need a heart to live" have surprising exceptions.

> Pointing out that penguins can't fly doesn't make the case that birds can fly stronger in any way.

I disagree. It's such a common rule that there's a long Wikipedia page for the exceptions[1], and the first photo is of penguins, labelled "penguins are a well-known example of flightless birds.".

If I knew nothing else about the topic, I would take it as evidence that it's common for birds to fly, otherwise that fact would have been unremarkable. Not hard proof of a universal quantifier, but a useful rule nonetheless.

[1] https://en.wikipedia.org/wiki/Flightless_bird

[SketchySeaBeast]

> There are exceedingly few things we can claim to apply everywhere, and even fewer we can "prove" to each other.

Yes, this is why hard and fast rules don't make sense, and why they should have "generally", "normally", or "mostly" attached to them.

If you have two categories of birds, one with those that fly and one that doesn't, having that second list doesn't make the first stronger. At some point that second list dilutes that first one so much that it doesn't make sense anymore.

If my rule is that "white guys are named Dave" does my building a list of every example of a Dave and non-Dave make my rule stronger? When does the "strong" nature of the rule get watered down sufficiently? Honestly, a list of hundreds of birds tells me that it's a weak rule and that the "birds fly" rule is wrong.