Water

Human, can only survive with water in river, lake, and etc but not from the pure form of water such as steam.

Our body is a form of dirty mixture, our body never a pure form.

Dirty water can go through a process to purify itself, and reach a pure form of water. the principle apply to human, we cannot purify our body as it is a dirty mixture in nature, however, we can purify our soul.

Sound not good a metaphor, yes, it is not a good one. BTW, you know what I mean.