This is what makes Plato a rationalist, as opposed to an empiricist in epistemology. Senses can only give you information about the ever-changing and imperfect world of phenomena, and so can only provide you with implications about ultimate reality, not reality itself. Reason goes straight to the idea. You remember, or intuitively recognize the truth, as Socrates suggested in the dialog Meno . According to Plato, the phenomenal world strives to become ideal, perfect, complete.
Ideals are, in that sense, a motivating force.
In fact, he identifies the ideal with God and perfect goodness. If the world is not perfect, it is not because of God or the ideals, but because the raw materials were not perfect. The soul includes reason, of course, as well as self-awareness and moral sense. Plato says the soul will always choose to do good, if it recognizes what is good. So, someone who does something bad requires education, not punishment.