It is crucially important to grasp the fact that a concept is an "open-end" classification which includes the yet-to-be-discovered characteristics of a given group of extents. All of man's knowledge rests on that fact.
As one more illustration, consider the issue of literary style. Some styles are praised as economical; the writer communicates a complex content by means of relatively few words. Other writers are prolix weighing our consciousness down with more units than the content requires. At the evil extreme of this continuum is the writer who deliberately flouts the crow-epistemology; he seeks to subvert the reader's consciousness by loading it methodically with more units than it can hold. For example, he gives you a seemingly endless sentence, with a jingle of qualifications, subordinate clauses, and parenthetical remarks, erupting in the middle, all of which you must plow through and try to retain while you are still holding the subject of the main clause and waiting for the verb. After a few pages of such prose, the reader's mind simply closes, and the words turn into meaningless verbiage.
Concepts do not pertain to consciousness alone or to existence alone; they are products of a specific kind of relationship between the two.
Concept formation and use is precisely the realm that is not automatic or infallible, but volitional. In order to conceptualize, a man must expend effort; he must engage in the kind of mental work that no stimulus can necessitate. He must struggle to relate, connect, process an ever-growing range of data - and he must learn to do it correctly.