The “blueness of the picture” discussed in Section 11.5 refers to the measurable amount of blue pigment (or other source of blueness), not to the degree of truth of the claim that blueness is present. That abstraction is expressed in Lojban using jei, which is closely related semantically to ni. In the simplest cases, le jei produces not a number but a truth value:

is equivalent to “truth”, and

is equivalent to “falsehood”.

However, not everything in life (or even in Lojban) is simply true or false. There are shades of gray even in truth value, and jei is Lojban's mechanism for indicating the shade of grey intended:

Example 11.38 does not imply that George is, or is not, definitely a criminal. Depending on the legal system I am using, I may make some intermediate decision. As a result, jei requires an x2 place analogous to that of ni:

Abstractions using jei are the mechanism for fuzzy logic in Lojban; the jei abstraction refers to a number between 0 and 1 inclusive (as distinct from ni abstractions, which are often on open-ended scales). The detailed conventions for using jei in fuzzy-logic contexts have not yet been established.