Imagine in an early stage of the evolution of humans, humans had no thoughts. In fact, humans didn't know fear, and had no thoughts. Later in evolution, fear found its way in the mind of humans, and as fear came, so did the thoughts, because thoughts were a reaction of humans to try and get a hold on these fears. I guess we could say these thoughts were an acceptable alternative to the fears. The theory is simple: thoughts are acceptable fears, fears that humans changed to be able to handle them. In a previous stage of the evolution thoughts didn't exist at all, because fears didn't exist yet. I don't believe this theory is correct, but it's a very useful point of view, a very useful tool to find the fears, because real fears are a bitch to get a grip on, because they hide very well in the mind.