|Metaphor is one of the most fundamental figures of speech. Much of our everyday language is metaphorical in nature; we just don't always recognize the implicit comparisons that our most common expressions are making. A metaphor is a comparison of unlike things that seeks to identify the shared qualities of the things. |
The critic I.A. Richards has provided a useful set of terms to describe the parts of a metaphor. The tenor is the "gist" or "point" of the metaphor, the phenomenon the metaphorical turn of phrase is trying to convey. The vehicle is the concept conveyed by the word or group of words that is used to make the explicit comparison.
The word "thirst" is used here metaphorically and in contrast with the literal thirst that Narcissus seeks to quench at the pool. The vehicle "thirst," the concept of desiring water, is used to convey the tenor of Narcissus's erotic desire for his reflection. His passion for his own image is a like a thirst. The metaphor is particularly effective because the object of both desires is in this case "in the water."