So too understand the eye of the soul: whenever it is fixed upon that upon which truth and reality shine, it understands and knows and seems to have intelligence, but whenever it is fixed upon what is mixed with darkness–that which is subject to birth and destruction–it opines and is dimmed, changes its opinions this way and that, and seems to have no intelligence.
Say that what gives truth to the objects of knowledge, and to the knowing mind the power to know, is the Form of Good. As it is the cause of knowledge and truth, think of it also as being the object of knowledge. Both knowledge and truth are beautiful, but you will be right to think of the Good as other and more beautiful than they. As in the visible world light and sight are rightly considered sunlike, but it is wrong to think of them as the sun, so here it is right to think of knowledge and truth as Goodlike, but wrong to think of either as Good, for the Good must be honored even more than they.
From The Good and the Allegory of the Cave by Plato
Intro to Ethics class (=