You can't tell how deep a puddle is until you step in it. When I am asked about my profession, I have two ways of answering. If I want a short discussion, I say that I am a mathematician; if I want a long discussion, I say that I try to understand how the human brain works. A long discussion often leads to further questions: What does it mean to understand "how the brain works"? Does it help to be trained in mathematics when you try to understand the brain, and what kind of mathematics can help? What makes a mathematician turn into a neuroscientist? This may lead into a metascientific discussion which I do not like par- ticularly because it is usually too far off the ground. In this book I take quite a different approach. I just start explaining how I think the brain works. In the course of this explanation my answers to the above questions will become clear to the reader, and he will perhaps learn some facts about the brain and get some insight into the construc- tions of artificial intelligence.