Mutability Key to Imperative Logic

The only reason imperative logic (a.k.a instructions) execute in sequence is that due to the presence of mutable stored values, the result is dependent on the evaluation order. Using your vocabulary, an "instruction" can (and a "rule" cannot) operate on mutable values. – Shelby Moore III

http://stackoverflow.com/questions/602444/what-is-functional-declarative-and-imperative-programming