Final answer:
The analogy used is the Analogy of the Cave, where the journey from the darkness of the cave into the light symbolizes the soul's ascent to knowledge and truth.
Step-by-step explanation:
The analogy used to clarify the nature of the soul is Plato's famous Analogy of the Cave, found in Book VII of the Republic. In this analogy, prisoners are chained inside a cave, facing a wall, seeing only shadows cast by objects behind them, which they believe to be reality. This represents the soul's predicament in the physical world. The soul's ascent into the intellectual world is likened to a prisoner's journey out of the cave into the sunlight, finally able to see the true forms of things rather than mere shadows. The sunlight represents truth and the Form of the Good, which is the highest understanding that the soul can achieve. Thus, the journey upwards symbolizes the soul's ascent to a higher state of knowledge and truth.