What is an illusory concept? This dictionary will denounce as illusory many concepts held in general esteem. I am not at a loss, you see, for examples. Nevertheless, it is not an easy matter to describe in general terms what distinguishes a useful from an illusory concept, a true one from a false one, a living concept from a dead one.These opposite characteristics already point to some sources of the difficulty. The mere appearance of usefulness cannot always be attributed to the same source, and it is not in every case a simple matter to determine a concept's falsity or deadness. It may be that the falsity was inherent, or else became attached to the concept as language underwent changes over a period of time and did not need to wait for a scientifically and critically advanced generation to discover it. A concept could also have been dead to begin with, or death can have overtaken it unnoticed in popular usage after a shorter or longer life. The boundaries cannot be sharply drawn because all these concepts are relative. The concepts absolute and phlogiston were false from the start; exact scrutiny could all along have discovered that they contradicted the facts of experience.
We could be tempted to repeat that
Not if he had them by the neck, I vow,Would e'er these people scent the devil.