Are apples good to eat? Usually, but some apples are rotten.

Do humans have ten fingers? Most of us do, but plenty of people have lost a finger and nonetheless qualify as "human".

Unless you descend to a level of description far below any macroscopic object - below societies, below people, below fingers, below tendon and bone, below cells, all the way down to particles and fields where the laws are truly universal - then practically every generalization you use in the real world will be leaky.

(Though there may, of course, be some exceptions to the above rule...)

Mostly, the way you deal with leaky generalizations is that, well, you just have to deal. If the cookie market almost always closes at 10pm, except on Thanksgiving it closes at 6pm, and today happens to be National Native American Genocide Day, you'd better show up before 6pm or you won't get a cookie.

Our ability to manipulate leaky generalizations is opposed by need for closure, the degree to which we want to say once and for all that humans have fingers, and get frustrated when we have to tolerate continued ambiguity. Raising the value of the stakes can increase need for closure - which shuts down complexity tolerance when complexity tolerance is most needed.

Life would be complicated even if the things we wanted were simple (they aren't). The leakyness of leaky generalizations about what-to-do-next would leak in from the leaky structure of the real world. Or to put it another way:

Instrumental values often have no specification which is both compact and local.

Suppose there's a box containing a million dollars. The box is locked, not with an ordinary combination lock, but with a dozen keys controlling a machine that can open the box. If you know how the machine works, you can deduce which sequences of key-presses will open the box. There's more than one key sequence that can trigger the button. But if you press a sufficiently wrong sequence, the machine incinerates the money. And if you don't know about the machine, there's no simple rules like "Pressing any key three times opens the box" or "Pressing five different keys with no repetitions incinerates the money."

There's a compact nonlocal specification of which keys you want to press: You want to press keys such that they open the box. You can w...