I just wanted to add a small point about the confusion your brain is having when you try this exercise. The biggest revelation for me was when I realized that the boxes don't actually have a "correct" configuration. Here is a quick example I did. I first did the two center boxes at 1 with no indication of how they intersect. Then I photocopied it four times and indicated the intersection four different ways with pictures 2-5. I started with the exact same two boxes but got four different configurations. All of them are "correct" simultaneously. This is why your brain freaks out. It can see all the possibilities. I think this is because we are drawing something abstract where size and distance are not determined by the object itself. There is no external reference. If you drew two people and one was significantly smaller and overlapped by another person, your brain would instinctively know which was forward and which was back. With this exercise, you are making the decision as to which configuration your going to show. My advice is to start with deciding on a single line of intersection. After that, the other lines are mostly predetermined. For example, in picture 4 I emphasized the top line of the box on the left. This then led to other lines emphasized and I ended up with what I ended up with. All the boxes started with a decision of which box would be forward, back or partially covered. Hope this helps.