4.1 Base types

No object satisfies more than one of the following predicates:

boolean?          pair?
symbol?           number?
char?             string?
vector?           procedure?
null?

These predicates define the base types boolean, pair, symbol, number, char (or character), string, vector, and procedure. Moreover, the empty list is a special object of its own type.

Note that, although there is a separate boolean type, any Scheme value can be used as a boolean value for the purpose of a conditional test.