line drawn on water

noun2

1. •
   (figurative) Something transient or ephemeral.

   • The anger of humble and saintly people is a line drawn on water. It never lasts for long. — 2009
2. •
   (figurative) An ill-defined or variable border between concepts or things.

   • The border between madness and mental balance is a line drawn on water. — 2000