Riddle 4

We do not exactly know what it is,

We say it is, but not what is thought,

It is different for another when it is true for one,

They are never the same, yet it is one,

It cannot be described, nor shown,

No matter how others try.

It cannot be true, as it can be altered,

It cannot lie, as it tells the truth.

What do you think?

By the way, this picture has no reference to the answer, I put it up, because it reflects the thought of life, how magnificent and mysterious it is, we don't really think at all, and just take it for granted. AGAIN THE PICTURE IS NOT MINE!!! I just used it for inspiration, click on the picture to see who actually made it.