Here’s the first paragraph of this lesson:

It is because the thoughts you think you think appear as images that you do not recognize them as nothing. You think you think them, and so you think you see them. This is how your “seeing” was made. This is the function you have given your body’s eyes. It is not seeing. It is image making. It takes the place of seeing, replacing vision with illusions.

In other words, what we see when we look around us is not reality but rather our interpretation of reality. Our perception of everything starts with our thoughts. What we first think, we then perceive.

In fact, the Course refers to this line from Proverbs: As a man thinketh, so does he perceive.

This lesson is helping us pay attention to our thoughts since all our sense of wellbeing (or lack thereof) comes from our thoughts.

And our thoughts are sourced from the mind. When we choose the ego mind, ALL our thoughts are related to fear, shame, attack, and defense. Conversely, when we chose the mind of spirit, we can only experience a reflection of love.