well, *the* indicates there is a referent; whereas *a, or an*, or mere plural without *the* indicates a mere concept. Of course, the notion of concept itself is a muddy one: it contains extension, and intenSion. The extension of concept is determined by the intenSion of the same: for instance, the intenSion of the concept Water is all substances that has chemical structure H20. Extension is set of objects that exist in the world, and whose essential characteristics are those given in the intension.
Sometimes, there exists a concept whose extension is empty set: we have a concept of phlogiston, but whose extension is empty set. Idem for devil. For Christians, the extension of devil is not empty; but for me, the extension of devil is empty. Here, *definite article* plays crucial role in saying what is intended.