The song [eX Dream](http://tw.myblog.yahoo.com/cathy910732/article?mid=1128&prev=1129&next=382&page=1) by Myuji (which appears [in an anime OP](http://en.wikipedia.org/wiki/List_of_X_episodes)) contains the sentence:

> 
> [夢]{ゆめ}は夢でしかない
> 
> 
> 

The general point of the sentence is fairly clear, meaning "a dream is just a dream". But what is the best way to think of the でしかない construction?

At first I saw it as 夢は夢で, as in, "a dream is a dream and...", but now I'm wondering how correct this is or if it's correct at all.

Is it better to look at it as a separation of the parts of である? Does this happen in many other cases, where you use a qualified で followed by ある?