Duchamp then devises a wonderful comparison between this almost inconceivable prospect of seeing an entire 3-D object all at once and something we can easily achieve with our analogous sensation of touch. Suppose we hold a small penknife firmly in one hand. We can touch the knife's outer surface at all points simultaneously, and we can reconstruct the form of the object from these sensations even though we cannot see all parts of the knife's surface simultaneously from any single vantage point in a 3-D world. Now, Duchamp suggests, suppose we regard our simultaneous touch in three dimensions as analogous to the possibility of simultaneous sight in four dimensions. Then we will understand how we might “see” the entire surface of a 3-D object all at once.