"For this is our central human problem: that we are almost constantly the victims of our emotions, always being swept up and down on a kind of inner-switchback. We possess a certain control over them; we can 'direct our thoughts' -- or feelings -- in such a way as to intensify them. This is certainly our most remarkable human characteristic: imagination. Animals require actual physical stimuli to trigger their experience. A man can retreat into a book -- or a daydream -- and live through certain experiences quite independent of the physical world. He can even, for example, imagine a sexual encounter, and not only experience all the appropriate physical responses, but even the sexual climax. Such a curious ability is far beyond the power of any animal."