To make this as simple as I can
at the moment: As far as I can see, all symbols have the same set of relationships to a reality as maps have to a terrain. Furthermore, all ideas or concepts about a reality are symbolic. That is, ideas or concepts are a subset of symbols. Hence, all ideas or concepts about a reality have the same set of relationships to a reality as maps have to a terrain.
The set of relationships that symbols, including ideas, can have to a reality (ideally) range anywhere from complete correspondence to complete non-correspondence. Moreover, the degree to which an idea significantly corresponds to a reality can be thought of as its "truthfulness". The degree to which an idea significantly does not correspond to a reality can be thought of as its "falsity". That is analogous to saying that the degree to which a map corresponds to a terrain can be thought of as its accuracy, and the degree to which it doesn't correspond can be thought of as its inaccuracy.
Thus, suppose we have two competing ideas. Let's call the first idea "A", let's call the second idea "B", and let's call the reality that A and B compete to most perfectly or completely correspond to, "R".
Furthermore, suppose for the sake of illustration that A consists of the notion that R is red, hard, and hot, while B consists of the notion that R is red, hard, hot, and throbbing. In other words, A is a less elaborate description of R than is B.
Next, for the two ideas to be in genuine or true competition, what we know about A's correspondence to R must be the same as what we know about B's correspondence to R. That is, A and B must have the same set of known correspondences. Say, for the sake of illustration, that set is "red, hard, and hot". Why the same set? Because if they had a different set of known correspondences, then one or the other idea would more completely correspond to R, and the two would not be in genuine competition (although some folks, speaking loosely, might still call them in competition).
Now, the rule of parsimony basically states that whenever we have two competing ideas about a reality, we go with the idea that is the simplest or most parsimonious. If we apply the rule to our illustration, then we would pick A over B as the most parsimonious idea. But picking A over B is logically the same as picking the idea which introduces the least unfounded speculation over the idea which introduces the most unfounded speculation, for we know that A's "red, hard, and hot" is less speculative than B's "red, hard, hot, and throbbing", since "throbbing" is not in our set of known correspondences.
Thus, to sum up all of the above, one reason someone might choose to apply the rule of parsimony to pick between genuinely competing ideas is in order to pick the idea that is least speculative. Of course, someone else might prefer the more speculative idea for personal reasons (e.g. they just darn well like the notion that R is "throbbing", and don't care that "throbbing" bears no known correspondence to R). The choice between parsimonious and greater complexity is somewhat a matter of taste.
At least that's how I see it. Of course, I just woke up from a nap, my head is still a little groggy, and so my thoughts on this matter are less than parsimonious, and probably garbage anyway.
