# What is the difference between a predicate and function?

I need to to understand the difference between predicates and functions in the context of Clasual Form Logic in order to define the Herbrand universe.

If I have p(x) :- q(f(x)) would I be right in saying that p and q are predicates while f is a function because it is "nested"? By this thinking then if I have p(x) :- q(x) both p and q are predicates and I have no functions?

If this is incorrect then how can I tell the difference between a predicate and function?

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A predicate is a box that takes an argument and returns a Boolean value. For example, "$x \mapsto x \text{ is even}$".

A function is a box that takes an argument and returns a value. For example, "$x \mapsto x^2$".

Edit (following Amy's suggestions): There is some domain over which all variables range. A function takes zero or more arguments of that domain and returns another argument from that domain. A predicate takes zero or more arguments of that domain and returns a Boolean value.

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By your definition, then, since a predicate returns a "value" (albeit a specific kind of value: Boolean), is it also a function, whereby a function includes predicates, but also includes "boxes" that return non-boolean values? Just hoping to clarify for OP. –  amWhy May 17 '11 at 19:40
I don't understand: if a function takes arguments in a domain D and returns another argument in D, then how about a function from R to Q? I think a good distinction is that a predicate is an expression that ascribes a property to some thing or things (to subject(s) in your choice of universe), and has a singleton as input (more than one input, you get a relation). A function from X to Y assigns a single value in Y to each value of X. –  gary Jun 19 '11 at 20:29