I am not so sure I would call it a metaphor, but I do think the word has attached to many many ideas which are not itself due to the fact it is a word for a think that cannot actually be communicated.
In the same way that it would be impossible to communicate the colour red (even though you could teach the word and even some concepts behind it) to a blind person, but we are not all blind to this soul idea, but its a blurry hard to grasp concept and its not made up of sense impressions so communication of it very difficult.