The metaphor is a transfer of the name, the use of words and expressions not on their direct appointment. All sayings and proverbs are metaphors, open before the person kind of secret sense which he has to guess or understand.
It is necessary for a metaphor that the person realized its effectiveness. When you speak "up to the soul depth", mean the spiritual phenomenon having nothing in common with space and deprived of such characteristics as a bottom or a surface by the word "depth". Designating "depth" as some particle of soul, everyone understands that this word is used not literally, and the necessary indirect sense is derivative of direct.
Why does the person use words not on direct purpose? Why does not prefer direct designation and does not use a word in the true sense?
Not only it is heavy to call the mental object interesting the person, but also it is difficult to understand. It escapes, it cannot be caught. The metaphor serves development of thinking, and not just the name of a subject. Deep function of a metaphor consists in knowledge. It is necessary not only to make a thought of the person available to other people, but also to the person in order that an object became more available to thinking. The metaphor is the important tool of thinking, and not just means of expression of a thought. Not all objects are available to thinking of the individual, not about all things he can make a clear separate idea. Your soul is forced to address easily available objects, having accepted them as a starting point and having made itself a concept about hardly perceptible, more difficult things. The metaphor is a tool of a thought by means of which the person will manage to reach the most remote sites of the conceptual field. It does not move apart borders imaginable but only provides access to those things which vaguely are seen at distant boundaries. The metaphor is widely used in poetry where its function is thoroughly studied. Approach a poetic and scientific metaphor from identical positions. It is considered as the bewitching flash which lit up the light. Do not apply a concept of the truth to it and do not consider the tool of knowledge of reality. And it does not prevent to notice that the research is not alien to poetry, and its methods are capable to open the same positive facts what are opened by science.