Why do people think perpetual motion cannot exist? If you believe, you can achieve?

The main reason, very abstractly speaking, is because matter and energy co-exist, they cannot be separated each from another. This implies that there is no ideal interaction, in every case it is involved an intermediate, which will cause dissipation. See also: a href="http://en.wikipedia.org/wiki/Entropy" rel="nofollow"http://en.wikipedia.org/wiki/Entropy/a