The preface begins with the authors explaining that every vertebrate has a blind spot in their retina called a scotoma. The area does not contain any light-sensitive cells and therefore has no means of transmitting light to the visual areas of the brain. “Paradoxically” (xi), the authors add, you are capable of detecting your own blind spot visually. They provide a grid with two black dots and a plus sign between them so you can see your blind spot in action. When you cover one eye and hold the book around 6 inches from your face, the dot on the side of your open eye disappears. Your brain fills the dot with a continuation of the grid, or “something that made reasonable sense” (xi). An even more extreme condition is known as blindsight, which is when the visual cortex is damaged. Patients with this condition can’t see an object directly in front of them but can still reach for it because certain pathways that dictate visual behavior are intact.

The authors directly compare these visual phenomena to “another type of blindspot” (xii) on which the book focuses: the hidden biases of which we are unaware—like the retinal scotoma in our eyes.