Page 309 - Deep Learning

292 Conversion

and when astronauts returned from space with photographs of a round earth
rotating in space, the knowledge that the earth is spherical became immedi-
ately accessible to everyone. The conversion from a universal belief that the
earth is flat to an equally universal belief that the earth is spherical happened
to the human species as a whole over the course of approximately two and a
half millennia.
individual human beings go through a shorter version of this process.
3
Young children have little doubt that the earth is flat. Parking lots certainly
suggest as much, as do the schoolyard and the local lake. Children gradually
come to grips with what adults mean when they say that the earth is round or
when they point to a colored ball in a bookstore or a library and say, “that’s the
earth.” empirical studies indicate that in the normal case, the shift from the
flat to the round earth takes several years. in contrast, many adults remem-
ber the specific day when they underwent another prototypical case of belief
revision: The passage from belief in santa Claus and his fantastic ability to
visit every child on one and the same night to the realization that the man
in the red coat is a department store employee. in adulthood, there are many
such opportunities to revise beliefs: Does this or that diet, exercise regimen or
health practice provide the benefits claimed for it? Has the evidence for global
warming reached a point at which it would be prudent to prepare for climate
change? Do the political principles that in the past seemed reasonable still
deserve allegiance? Do the religious precepts absorbed in childhood still make
sense in middle age?
The shared feature of these examples is that they confront a person with
alternative and incompatible beliefs. The earth is either like a pancake or like a
soccer ball; as we ordinarily understand those words, we cannot believe both.
Likewise, santa Claus cannot both be the creature of the fairy tale and also a
store employee. An exercise regimen either does or does not produce mea-
surable health effects. The average global temperature is rising or decreas-
ing or keeping steady; if it is changing, it moves either upward or downward.
To adopt one of these beliefs is to abandon, disbelieve or reject the alternatives.
The central question in this and the next chapter is how, by what cognitive pro-
cesses, a person comes to believe what he previously disbelieved or vice versa,
a process variously labeled belief revision, conceptual change, restructuring and
theory change in different niches of the cognitive sciences. The received view
claims that people adopt beliefs that are supported by evidence but reject or
revise those that are contradicted by evidence. The rejection of that view in
this chapter prepares the ground for the different view of belief formation and
belief revision proposed in Chapter 10.