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208 Adaptation

a candle flame, but a picture of a candle does. Similarly, if a chess player loses
a match, what is the lesson? A useful answer cannot refer to the specific moves
made in that match, because the match is over and will never be played again.
Exactly which change in the player’s strategy is implicated by the disappoint-
ing checkmate? In general, if action A generates an erroneous or undesired
outcome in situation S, how can the person’s brain compute the class of situa-
tions {S} in which A should be avoided? We are demonstrably capable of learn-
ing from our errors, so our brains must be able to carry out a computation of
this sort. The micro-theory of learning from error presented in this chapter
specifies such a computation. 9

ERROR DETECTION

The triggering condition for learning from error is the occurrence of an error,
so the first two questions are what we mean by error and how people detect
their own errors. A third question is what information an error provides that
the learner’s brain can use to compute the indicated revision of the relevant
practical knowledge.

Objective Versus Subjective Errors
A driver approaches an intersection and makes a left turn; his intended desti-
nation required a right turn. We recognize this as a mistake, but there are com-
plexities. It cannot be the case that the action type turn left is erroneous in any
absolute sense. There are a good many intersections at which turn left is cor-
rect. Rather, it was the occurrence of that particular token of turn left in that
particular intersection that was mistaken. The fact that the driver’s destination
lay to his right is one of the facts that made the left turn erroneous. Another
relevant fact is that the driver approached the intersection from the south. If
he had approached the same intersection from the north, with the same desti-
nation in mind, a left turn would have been correct. Whether the left turn is an
error also depends on the availability of alternatives. It was a mistake in part
because the driver made a left turn instead of a right turn. If the other roads at
the relevant intersection were blocked by roadwork so that the left turn was
the only action available, then it would not have been an error. It might be the
start of an undesirable detour, but if the left turn was the only physically possi-
ble action, it was nevertheless not a mistake. The possibility of error is a conse-
quence of the fact that most situations present a person with multiple options,
only some of which are on any path to the desired goal.