Plato's divided line theory illustrates the distinction between the different levels of knowledge and reality. This theory asks us to imagine a vertical line. The left half is dedicated to metaphysics (the study of being), and the right half is dedicated to epistemology (the study of knowing). Now imagine a horizontal line running through the middle of the vertical line, to form quadrants. The upper left quadrant represents the intelligible world of being which is invisible, eternal, unchanging, indivisible, and immaterial. The lower left quadrant represents the visible world of becoming, which is visible, temporal, changing, divisible and material. The upper right quadrant represents knowledge (knowing is the state of mind dependant upon being). The lower right quadrant represents opinion.
Next, we must imagine segments on the vertical line from lowest (D) to highest (A). On the left (being) side, the segments are degrees of reality, and on the right (knowing) side the segments are degrees of certainty or clarity.
The lowest level (D) represents illusions in the visible world of becoming quadrant on the being side. (D) Represents imagination in the opinion quadrant on the knowing side. At this level we tend to carry illusions and form opinions based on shadows, images, reflections, pictures, fantasies, etc.
Ascending to the next level (C), we have belief in the visible world of becoming quadrant on the being side, and perception in the opinion quadrant on the knowing side. At this level, we actually perceive the objects themselves unlike imagination where we observe reflections, pictures and shadows of things, or experience illusions and fantasies. The objects that we see at the perception level are physical and three-dimensional such as a tree, but we are still unable to make the distinction between different types of trees. A common saying that illustrates this level...