Most of this is specific to videogames and probably will not be applicable anywhere else:
An emergent property in the context of videogames is one the designers of the game did not intend, [more strictly: yet is not a programming error].
Excluding the possibly, since this example is ambiguous using it:
In the game Super Smash Bros, jumping is not emergent, since the designers programmed it into the game specifically.
Wavedashing [dodging into the ground so that you will be able to move while attacking] (and in fact, every single bit of strategy for every character) is emergent; it's not programmed into the game, it's just that if you put together all the intended rules of the game, wavedashing appears also.
What does "this is emergent" tell you, in this context?
It tells you first of all it's unintentional, which then tells you it has a vastly greater chance of being unbalanced or broken.
Using the stricter definition, it also tells you whatever it is profits the player in some way, because if it did not profit the player in some way it would not have emerged; someone would have found it, not used it or told anyone, and it would just fade away. (But this is only valid for emergent things when they're structured in a certain way. This part can be generalized to, say, the economy, but not, say, to traffic jams, because traffic jams are more tragedy of the commons types of things.)
It also tells you, most importantly, that it is probably not possible to know all the specific causes of this thing and instead to try wide and general causes. (i.e: "World War I happened because of a general attitude among nations that military force was a good way to solve problems." It's possible to say it happened "because Alice thought... and Bob thought... but Carol thought... and Dave thought....", but this is going to be either much less accurate or not worth the effort to make it accurate.)
What does "this is non-emergent" tell you?
There are one or more obvious specific causes that it would not be worth breaking down. (In the case of videogames, the developers, but it also works for cases like "there is a big dent in the front of my car because I crashed it into a tree" [but wait, you say, isn't that also phrasable as "because the force from the tree caused this molecule and this molecule and this molecule to move backwards"? Yes, but it doesn't matter; the only cause is still the force from the tree.])
(Finally, random other example I thought of after reading the Go example:
In chess, the position of the pieces at the beginning of the game is not emergent: there is one cause for that: because it's part of the rules of the game.
The fact that the best first move for white in most cases is pawn to e4 is emergent. Nobody wrote that into the rules of chess; it's just a consequence of the positions of the pieces.)
What's the difference between a "programming error" and an "emergent consequence of the program as written", other than whether the programmers decide they like the result? Is it just a question of whether the rules involved can be described intuitively at the level of user-interface objects rather than lines of code?
The failures of phlogiston and vitalism are historical hindsight. Dare I step out on a limb, and name some current theory which I deem analogously flawed?
I name emergence or emergent phenomena—usually defined as the study of systems whose high-level behaviors arise or “emerge” from the interaction of many low-level elements. (Wikipedia: “The way complex systems and patterns arise out of a multiplicity of relatively simple interactions.”)
Taken literally, that description fits every phenomenon in our universe above the level of individual quarks, which is part of the problem. Imagine pointing to a market crash and saying “It’s not a quark!” Does that feel like an explanation? No? Then neither should saying “It’s an emergent phenomenon!”
It’s the noun “emergence” that I protest, rather than the verb “emerges from.” There’s nothing wrong with saying “X emerges from Y,” where Y is some specific, detailed model with internal moving parts. “Arises from” is another legitimate phrase that means exactly the same thing. Gravity arises from the curvature of spacetime, according to the specific mathematical model of General Relativity. Chemistry arises from interactions between atoms, according to the specific model of quantum electrodynamics.
Now suppose I should say that gravity depends on “arisence” or that chemistry is an “arising phenomenon,” and claim that as my explanation.
The phrase “emerges from” is acceptable, just like “arises from” or “is caused by” are acceptable, if the phrase precedes some specific model to be judged on its own merits.
However, this is not the way “emergence” is commonly used. “Emergence” is commonly used as an explanation in its own right.
I have lost track of how many times I have heard people say, “Intelligence is an emergent phenomenon!” as if that explained intelligence. This usage fits all the checklist items for a mysterious answer to a mysterious question. What do you know, after you have said that intelligence is “emergent”? You can make no new predictions. You do not know anything about the behavior of real-world minds that you did not know before. It feels like you believe a new fact, but you don’t anticipate any different outcomes. Your curiosity feels sated, but it has not been fed. The hypothesis has no moving parts—there’s no detailed internal model to manipulate. Those who proffer the hypothesis of “emergence” confess their ignorance of the internals, and take pride in it; they contrast the science of “emergence” to other sciences merely mundane.
And even after the answer of “Why? Emergence!” is given, the phenomenon is still a mystery and possesses the same sacred impenetrability it had at the start.
A fun exercise is to eliminate the adjective “emergent” from any sentence in which it appears, and see if the sentence says anything different:
Another fun exercise is to replace the word “emergent” with the old word, the explanation that people had to use before emergence was invented:
Does not each statement convey exactly the same amount of knowledge about the phenomenon’s behavior? Does not each hypothesis fit exactly the same set of outcomes?
“Emergence” has become very popular, just as saying “magic” used to be very popular. “Emergence” has the same deep appeal to human psychology, for the same reason. “Emergence” is such a wonderfully easy explanation, and it feels good to say it; it gives you a sacred mystery to worship. Emergence is popular because it is the junk food of curiosity. You can explain anything using emergence, and so people do just that; for it feels so wonderful to explain things.
Humans are still humans, even if they’ve taken a few science classes in college. Once they find a way to escape the shackles of settled science, they get up to the same shenanigans as their ancestors—dressed up in the literary genre of “science,” but humans are still humans, and human psychology is still human psychology.