Hi Lina!
Some hints to help you in your quest for a main idea.... this is one of those verbose philosophy passages, where the main idea is hidden amongst a lot of language. The author suggests early on that in the animal, sensibility and intelligence function towards survival. But, in the human, intelligence has become separated from those survival goals. In fact, in the human, though can actually interfere with survival (for example, fair play is a great idea, but may be detrimental to an individual's survival).