The most basic kinds of things the logic works with are “true”,
“false”, and individuals. “True” and “false” are written as
F. The individuals you will usually see in Prooftoys are numbers.
All the rest are functions. Even collections in Prooftoys are
The values true and false, written
F, are known as truth
values. These are often referred to as boolean values in honor of George
Boole, a pioneer in the development of these ideas.
Prooftoys takes “not”, for example, as a function that has a truth
value as input and a truth value as its output. In Prooftoys and most
logics it is written as “¬”. Suppose we say that x is not less then
¬(x < 10). In this example if
x < 10 is true,
¬(x < 10) is
x < 10 is false,
¬(x < 10) is true.
Similarly, “and” is taken as a function from two boolean values to a
boolean value, and is written as “∧”. So we express that idea that x
is greater than zero and less than 10 by writing
x > 0 ∧ x < 10.
This whole statement is true if it is true that
x > 0 and it is also
x < 10. If either of them is false, the whole statement
is false. Discussion with pictures here.
All values in Prooftoys are boolean values, individuals, or functions.
Functions such as addition and multiplication have inputs that are numbers (individuals) and outputs that are also numbers.
A predicate tests a value and gives a true/false result. For example, a test whether a number is even is a predicate. Every predicate can be thought of as a collection. In Prooftoys there is no technical difference between “the set of all positive numbers” and the predicate that tests whether something is a positive number.
A relation such as less than tests whether a number is less than
another number, for example
x < y. It has two inputs and gives a
So predicates and relations are functions that produce values that are
false, and a predicate can also be thought of as a
A function can also have functions as inputs and even functions as outputs.
Collections are functions whose values are boolean, so there is no technical difference between a collection and a predicate or relation.