The success of camouflage depends on how its viewer sees the world at any given moment. A viewer can see with secondary vision, which means she looks at the environment with specific, preconceived notions as to what she might find within it -- say, a deer, a man or a vehicle. The viewer's mission is to search and understand the world around her by putting it into context of what she expects to see. Secondary vision, however, is limiting in this respect, because if any given object or sight does not correspond with what the viewer is already looking for, it can go unnoticed. That said, if a viewer observes her surroundings using primary vision, she is much more likely to spot people, animals and things that might be hidden. Primary vision allows a viewer to break her world into parts -- instead of looking for the whole object, the viewer will look for pieces of that object, fragments that might be scattered over or within a landscape. This allows a viewer whose brain might otherwise "dismiss" certain objects as being insignificant to, instead, find her targets by picking up their individual pieces.
The human brain tends to think in terms of categories. When it comes to seeing, the brain categorizes the world according to such terms as color and shape. When we are looking for a deer in the woods, what we are really looking for is an animal that is deer-like in shape and color. In this way, camouflage again takes advantage of how our brain operates, as most camouflage works to disrupt the shape and color of the object it's hiding, making a soldier, for example, look less like a soldier. Military camouflage swathes soldiers in contours, shapes and colors that are not only foreign to the human form, but are more akin to the natural surroundings in which those individuals operate. Your shape and color are broken down by his camouflage, and when a viewer looks upon you with secondary sight, he will thus not notice you.
Another reason why camouflage is so successful to its wearer is found in our brains' constant search for continuity. Objects that are similar in shape and color tend to be grouped together as a single unit, which is why life jackets are usually bright orange -- this color stands out against the blue, brown or green of the water around it. If the jacket were cooler in color, it (and the person inside it) might simply be blurred in with the waves. On the flip side, if you wanted to blend in with the water -- or the trees or the beach, for that matter -- imitating the shapes and colors of those surroundings could easily help you to become grouped in with the environment and lost in the landscape.
When you scan your environment in search of something, you're looking for patches of positive space, or the space that is filled by an object. Negative space is the empty space which surrounds that object. Your brain is naturally drawn to positive space as a focal point, since you are almost always looking for something as opposed to nothing. Camouflage makes use of both positive and negative space: When you want to blend in with an object, you disguise yourself with features from an area of positive space, including texture, color, shade and shape. However, sometimes it is far more effective to conceal oneself within negative spaces, if only because there are more of them. So if you're on the ocean and don't want to be seen, you might want to blend in with the ocean itself rather than the dinghy in which you're riding. In the end, camouflage isn't about looking like something else as about looking like nothing in particular.