The Definitions of 'Business Rule' and 'Rule'

by Ronald G. Ross

In this one-per-month special 'notepad' series, I am taking a quick look at important issues facing practitioners who are seeking to understand and apply the business rule approach for capturing business requirements and developing business systems.

In the business rules community, there is fairly widespread acceptance of the following definition for 'business rule' from the the seminal paper on business rules, produced by the Business Rules Group (BRG) in 1995: 

"...[A] statement that defines or constrains some aspect of the business... [which is] intended to assert business structure, or to control or influence the behavior of the business.  [A business rule] cannot be broken down or decomposed further into more detailed business rules ...   [I]f reduced any further, there would be loss of important information about the business."^[1]

Loosely, 'defines' in the BRG definition implies terms and facts ('structure [of knowledge]').  'Constrains' implies rules ('control or influence').  So generally, business rules implies 'terms, facts, and rules,' the definition used by Ronald G. Ross in The Business Rule Book.^[2] 

Aside:  The BRG definition alludes to, but does not specifically say 'atomic form of rules.'   There is a very solid, precise definition of 'atomic rule form' in formal logic.^[3]

The BRG definition is accepted in all three major-publisher books to date:

1. Business Rules Applied:  Building Better Systems Using the Business Rule Approach^[4] by Barbara von Halle

2. Business Rules and Information Systems^[5] by Tony Morgan

3. Principles of the Business Rule Approach by Ronald G. Ross

Another Aside:  As part of its work-in-progress, the BRG intends to provide a more exact, business-level definition of 'business rule' suitable for semantic models.  It will not be exactly 'terms, facts, and rules,' but it will build on, and provide greater clarity for, these basic ideas.

What about 'rule'?  From a formal point of view, this question has been determined by formal logic for quite some time.  The following^[6] explains:

[Rules from a real-life perspective] are the IF-THEN connective of predicate calculus (also known as the implication connective or logical implication).  The general form of the IF-THEN connective appears below.

IF p THEN q

Note the following about this form.

• p and q must be Boolean expressions, that is, they must evaluate to either true or false.
• p is called the antecedent.
• q is called the consequent.
• This example can also be read as follows: "p implies q."

In predicate calculus, the expression IF p THEN q is exactly equivalent to the following logical expression, which might actually be easier both to understand and to interpret correctly.

OR ((NOT p), q)

The syntax is unimportant here.  This expression could also be written ((NOT p) OR q).

This expression is exactly and precisely the fundamental formal definition of a rule.  What the expression says in everyday English is simply that "Either p is not true or q is true." 

So when can p be true?  It can be true only if q is true.  In other words, the truth of q is required for p to be true.  If q is not true, then p cannot be true.

Here's an example to illustrate.

p:  "It is raining"

q:  "Streets are wet"

Rule:  OR ((NOT "it is raining"), "streets are wet")

Question:  When can "it is raining" be true?

Answer:  Only if "streets are wet" is true.

If "Streets are wet" is not true, then according to the rule, (NOT "it is raining") must be true.  The only way that (NOT "it is raining") can be true is if "it is raining" is false.

References

[1]  Business Rules Group, Defining Business Rules ~ What Are They Really? (formerly known as the "GUIDE Business Rules Project Report," 1995), 4th ed. (July 2000), pp. 4-5.  Available at www.BusinessRulesGroup.org. 

[2]  Ronald G. Ross, The Business Rule Book, Business Rule Solutions LLC, 1997, pp. 241-246. 

[3]  Ronald G. Ross, Principles of the Business Rule Approach, Addison-Wesley, 2003.  Available though www.BRSolutions.com. 

[4]  Barbara von Halle, Business Rules Applied:  Building Better Systems Using the Business Rule Approach, 2002. 

[5]  Tony Morgan, Business Rules and Information Systems, 2002. 

[6]  Excerpted from Principles of the Business Rule Approach, pp. 231-233. 

standard citation for this article: Ronald G. Ross, "The Definitions of 'Business Rule' and 'Rule'," Business Rules Journal, Vol. 4, No. 4, (April 2003), URL:  http://www.BRCommunity.com/a2003/b140.html.