Simple is an adjective to refer to the things that have a property being defined or consist of a small number, (the number of parts, or especially, the number of (different) (inter)dependencies).
It's all about this number.
That is to say, a subject is more difficult, as it has more interdependences, and more simple, as there are less interdependencies. If something is defined by linear dependency, it would be considered simpler than something that is defined by non-linear dependences. However, if there is a large number of linear dependences, which interdepend, it might be considered more difficult because of larger number of interdependences.
Something might be also considered difficult or simple just because of multitude of "parts" (for example, when we say, "this physical exercise is very difficult", we might mean the multitude (magnitude) of force needed to apply", when we say "this math exercise is simple", we might be refering to number of interdependencies, when we say "this problem is difficult to solve", we might be refering to number of calculations or computing cycles to be done to solve it).
Problems that have many same type interdependences, not necessarily are more difficult than something that has fewer, but different type of interdependences, as it might require larger amount of time to solve them. In this case, it refers to the number of seconds (amount of time) needed to solve the problem, and not necessarily means that there is very large number of interdependences.
A simple dependency (function) has a small number of (inter)dependences in it, a small number of varialbles.
As "small" is a relative concept, "simple" is also a relative concept, and defined by the averge depending on subject we are talking about.
"Simple" also has a meaning of "easy to understand", which means it takes small number of seconds to become able to make correct predictions about a system. In this case, the number of interdependences is implied, as equations with a smaller number of (inter)dependences are easier to solve, and thus it's easier to predict the system.
Inyuki 02:47, 22 July 2006 (UTC)