Have you ever gotten up for the day, flipped on your Hi Fi and thought it sounded especially good today? I have and probably most of you have too. Now imagine that the night before you had just installed a new mains power cord for your amp. Voila, you're a believer in mains power cords. We are obviously highly vulnerable to this type of faulty judgement when assessing the differences in Hi Fi equipment.
If you buy a piece of equipment and it actually has zero impact on the sound of your Hi Fi, but you are (wrongly) convinced that your Hi Fi now sounds better, was it still a good purchase? After all, you're happy.