This is how I see it. When we were innocent it seemed simple. We could see the world clearly; things were how we saw them and there was always a correct answer to questions. But there was a problem. Other people did not give the same answer when it was obvious that they should. Fast forward to today. It has become clear that we do not see the world clearly, every one sees things differently and answers cannot be expected to be the same. Many observations have lead to this, for example consider the task of putting a small number of wooden blocks into a pile. So simple a two year old can do it. When attempts were first made to make a machine that could perform this task, it turned out to be any thing but simple. The machine had to know what a block was before it could see it and had to know a lot of things about the physical world so that, for example, it would know that it had to start with the block at the bottom of the pile and not the one at the top. Studies like this show that what we see is heavily dependent on prior knowledge, and give us huge respect for the mental abilities that a child has gained in its first two years. Other studies have uncovered an abyss of uncertainty.