Formal Logic/Predicate Logic/Informal Conventions

From Wikibooks, open books for an open world
Jump to navigation Jump to search
← Free and Bound Variables ↑ Predicate Logic Models →

Informal Conventions[edit | edit source]

Our predicate language, , like , is cumbersome and difficult to read. The superscripts and subscripts are distracting, and there are more parentheses than are needed. As with sentential logic, we will informally use simplified variants. The official language, however, is still used for definitions and other formalities. Most of the rules for generating informal variants will be familiar from sentential logic.

Transformation rules[edit | edit source]

We create informal variants of official formulae as follows. The examples are cumulative.

  • We drop a subscript if it is '0'. Thus we write instead of and write instead of . We cannot drop the subscript from , however.
  • The official grammar requires operation letters and predicate letters to have the superscript indicating the number of places. For example, is a two-place operation letter and is a three place predicate letter. In most cases, we can drop the superscript and let the context show the number of places. For example, we can write
Here we observe from the context that the operation letter has two places, thus leaving it understood that is an informal variant of . Similarly, we observe from the context that the predicate letter in
has three places. This makes , as used in this context, an informal variant of . This convention assumes our to be grammatically correct. In general, we will avoid grammatically incorrect expressions. We will also try to avoid, for example, using and in close proximity. Otherwise, their informal variant could cause confusion.
  • We will omit outermost parentheses. For example, we will write
instead of
  • We will let a series of the same binary connective associate on the right. For example, we can transform the official
into the informal
However, the best we can do with
  • We will use precedence rankings to omit internal parentheses when possible. For example, we will regard as having lower precedence (wider scope) than . This allows us to write
instead of
However, we cannot remove the internal parentheses from
Our informal variant of this latter formula is

Precedence and scope[edit | edit source]

Precedence rankings indicate the order that we evaluate the sentential connectives and quantifier phrases. has a higher precedence than . Thus, in evaluating

we start by evaluating

first. Scope is the length of expression that is governed by the connective. The occurrence of in (1) has a wider scope than the occurrence of . Thus the occurrence of in (1) governs the whole sentence while the occurrence of in (1) governs only the occurrence of (2) in (1).

The full ranking from highest precedence (narrowest scope) to lowest precedence (widest scope) is:

    highest precedence (narrowest scope)
    lowest precedence (widest scope)

Quantifier phrases have the same precedence as negation signs.

Examples[edit | edit source]

The following examples are predicate logic variants of the examples at the sentential logic Informal Conventions page. First,

can be written informally as


can be written informally as

Some unnecessary parentheses may prove helpful. In the two examples above, the informal variants may be easier to read as


Note that the informal formulae

are restored to its official form as

By contrast, the informal formula

is restored to its official form as