I thought a simile had to be something unrelated to the thing you're comparing it with. If I won the lottery I might say, "Wow! I'm richer than Bill Gates!". Surely that's not a simile?? Mmm.
I've always considered metaphors and similes as a subset of analogies, being comparisons made between things that aren't 'directly comparable' for some reason (don't come from the same source, can't be measured with the same scale, are very obviously different in ways that aren't mentioned, etc). I.e. they're figurative rather than literal comparisons. Jud - your Bill Gates example is not a simile; it's a direct literal comparison. You're comparing your monetary wealth to Bill Gates'. If you said something like, 'With such a lovely pair of twins to take home from the hospital, my partner and I are richer than Bill Gates!', you'd be using simile, as you're comparing wealth in babies to wealth in money (I'm not sure on the exchange rate there. It's a while since I've played the market.). Or if you won the lottery and said, 'Wow! I'm richer than a triple-chocolate mud cake!' - you're playing with two different definitions of the word 'rich' via comparison. You've probably come across the 'failed simile/metaphor for comic effect', where someone doesn't have the imagination to come up with something to compare to that is actually in a different realm. 'I'm happier than a... really happy person.' Although... what do people think of 'Happier than a pig in shit'? Is that a simile? If so, and if my above prattle is correct, doesn't that imply that the happiness a pig derives from rolling in excrement is actually different to the happiness I derive from rolling in excrement winning a card game? I'd have thought that it was the same type of happiness (same brain chemicals, etc), although maybe this goes back to my first paragraph: they could perhaps be measured with the same scale, but don't come from the same source. So I suppose several criteria can make for an analogy, and you need only meet one to qualify? In evolution, we talk about analogy and homology. Homology is when organisms share traits because they've inherited them from a common ancestor (e.g. humans and monkeys have directly comparable eyes because they're the same eyes that the common ancestor had). Analogy is when organisms have very similar traits, but they're not from a common source; they've evolved independently because they're traits that inherently work (e.g. humans and squid have lens-based eyes that are very similar in structure, but each evolutionary line figured that structure out for itself; they're not actually the same thing, but there are an awful lot of similarities). Maybe that last paragraph muddies the water more than anything, but I can see the analogy between the analogies Sorry for the obnoxious post; another one where I meant to write two short sentences but my wandering brain had other ideas...
I don't think there's any requirement for the things to not be directly comparable in a simile. I'm happier than a very happy person is still a simile, just not a particularly creative one. Happier than a pig in shit is also a simile. An analogy is a comparison that's used to transfer meaning (bit of a simplification, but I think that largely covers it. IIRC, pretty detailed article on Wiki). It definitely feels like there's some overlap with metaphor, but it doesn't feel like a sub/superset to me. E.g. something like this: Look, if someone gives you a choice between a Porsche and a donkey-cart, you know which one you'd pick, right? So when you're given the choice between no-name washing-up liquid and Sudso's sleek, high-power formula, there's only one option. There, we've got a comparison between the obviously good and obviously bad, and using the fact that most people would go for the Porsche to say most people should go for the washing-up liquid we're trying to sell. There's no real connection between the two use cases, but we're trying to transfer meaning - so it's an analogy. @Sifunkle's thing about evolution is an analogy, because 'similar traits with no common source' is like how literary analogies work as well.
This thread would suggest the three techniques (four if you include metaphor) are not quite as black and white as you may think. In fact they're about as black and white as a rainbow. Poor old idiom seems to have been left out. I'd have said 'Happier than a pig in shite' was an idiom, but @NigeTheHat says simile.
@NigeTheHat - so a simile is basically just any explicit comparison then? 'I'm as happy as I've ever been' would be a simile, as you're comparing your current happiness to all previous instances of happiness? I always thought it had to be a figurative comparison. @OurJud - I think 'idiom' is a word used colloquially to mean any catchy little phrase, but I think technically it has to be idiomatic, in that the words don't make sense in themselves; you need to 'just know' what the phrase means. I think it's common knowledge that pigs enjoy wallowing in filth, but perhaps that phrase would be idiomatic to someone who was completely unfamiliar with pigs.
As previously mentioned... 'Happier than a pig in shit' is both. It's a simile because it's a comparison, it's an idiom because it's a common phrase. Just like 'clear as mud'. Simile and metaphor are pretty distinct. I think there's definitely some blurring between metaphor and analogy, and idioms can also be one of the others (I guess... I can't immediately think of an analogy that's an idiom, but I don't see why something couldn't technically be both...) I think it can be any direct comparison, but I don't claim to be an authority. Just a guy who's remembered some stuff from English lessons. I thought it was any commonly-used phrase within the culture, but see above point about authority.
I'm not sure that's right - Wiki article suggests that idioms can have the literal meaning as the widely understood one, though figurative is certainly more common. Article here: https://en.wikipedia.org/wiki/Idiom But the key bit is:
But that's kinda the point, though. I wouldn't expect 'waste not want not' to need to be explained to anyone, the meaning's pretty clear. But according to that article, it's an idiom because it's a common phrase that's treated as a unit.
