A Long Time Ago
Once, the human body was quite different from the modern version. For a start, everyone had a tail: short and curly, or long and bushy. The management of the tail was of major importance. It would wiggle or sway when the owner was happy, and it gave excellent quick access to a person’s mood. When human beings evolved so as to wear garments, the tail was a key determinant on costume design: the pleats or folds of the outfit had to accommodate the tail and (for the most part) display it to advantage. There was a whole tradition, in those far-off days, of tail adornment: sticks and pebbles at first, then plaited grass, then crystals, then metal bracelets. There was a practice of ritual scarification and amputation in the more isolated tribes: a partially severed stump was a symbol of valour, a split tail was a sign of status and control.
As the human race evolved, it began to feel uncomfortable with its tail. It showed too much, it gave too much away. So dress codes were modified accordingly. Both genders began to adopt the crinoline: a swaying construct like a lampshade began to be de rigeur for the more polite groups, as it concealed the wagging or limp tail. But the crinoline could be inconvenient for daily life- you couldn’t get into a carriage or ride a horse in one. So folk began to create other concealing devices: the tail could be strapped to the thigh (like a sort of Prince Albert) or put into a little bag (like a codpiece).
All this became rather cumbersome, and over the years the human body evolved so as to get rid of the tail altogether. With most people, it became vestigial (though it could produce backache in some). But everyone remembered their tail, even at an unconscious level. And it functioned still: the Tail of the Mind.