Predicates that produce side effects leave the realm of pure logic. These are for example:

• writeq/1
• read/1
• format/2

Side effects are phenomena that cannot be reasoned about within the program. For example, deletion of a file or output on the system terminal.

Predicates that reason about instantiations are called meta-logical. Examples are:

• var/1
• ground/1
• integer/1

These predicates are outside the realm of pure monotonic logic programs, because they break properties like commutativity of conjunction.

Other predicates that are meta-logical include:

• arg/3
• functor/3
• (=..)/2

These predicates could in principle be modeled within first-order logic, but require an infinite number of clauses.

Predicates that reason about all solutions are extra-logical. These are for example:

• setof/3
• findall/3
• bagof/3

Predicates that impede or prohibit a declarative reading of Prolog programs are extra-logical. Examples of such predicates are:

• !/0
• (->)/2 and if-then-else
• (\+)/1

These predicates can only be understood procedurally, by taking into account the actual control flow of the interpreter, and as such are beyond the realm of pure logic.