This Side of the Glass

When does the light become the dark?
When does life become death?
Such polar opposites, yet, how is the change marked?
A spider's thread is the only separation, broken with a breath.

When does the right become wrong?
When does hate become love?
So similar are these thing when written in song,
And should we refuse one, how can we accept the one above?

When I look in a mirror, can I really be sure which side of the glass is me?