Halakhah do Ezechiela 3:27

And about the reason that water purifies everything impure, I would think on the side of the simple understanding that it is in order that a man see himself with the immersion as if he is created at that time, [just] like the whole world was water before man was upon it - as it is written (Genesis 1:2), "and the spirit of God floated upon the face of the waters." And through this comparison, he place upon his heart that [just] like he is renewed in his body, he also renew his actions for the good, fix his actions and be exacting in the way of God, blessed be He. And therefore the Sages said that the purification is not fit with water that is in a vessel, but rather only with living water - or collected [water], which is on the ground and, in any case, not in a vessel - in order to place in his heart the thought as if the world was entirely water, and [that] he is renewed with his emerging from them, as we said. But if the water was in a vessel - or even if it passed through a vessel - this matter that we said would not be set in the thought of the one immersing. As there is a limit to all that is in a vessel, which is the creation of the hands of man. And therefore when he immerses in a vessel, he will not think as if the whole world is water like at the beginning [of Creation], and that he is renewed at that time. 'And the one who accepts, will accept; and the one who refrains will refrain.'