Where have you been, Wolfgang?

yes - people can be united around a positive idea or a negative idea. This happens all the time, whether its a nation or a knitting club.

Yes, idealism has a dark side - as many cults, religions, and political movements have shown.

i wonder, does that mean that idealism can be fanaticism, or obsession? or are these things perversions of idealism?

i guess the question is - what is the ideal being worked towards?

here are a couple of dictionary definitions of idealism -

i·de·al·ism (-d-lzm)
n.
1. The act or practice of envisioning things in an ideal form.
2. Pursuit of one's ideals.
3. Idealized treatment of a subject in literature or art.
4. Philosophy The theory that the object of external perception, in itself or as perceived, consists of ideas.
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