Positivism (or, Ernst Mach-style positivism) works like this - you can only know what you observe. Therefore, you should only make scientific judgements off of observable quantities. Any theory that deals with a quantity that has not yet been observed is therefore invalid.

Makes enough sense. So why is positivism so reviled in the scientific community?

After all, one of Mach's disciples, Albert Einstein, used the idea of positivism in the special theory of relativity; he proved that you could not say that two events are simultaneous, because no measurement could be made that would provide the same result for all observers. So the person considered to be the smartest man in recent times was a positivist; again, why the badmouthing?

Because even Einstien said that positivism was a good tool, but a very, very bad, no-good philosophy. Use it as a tool, and see Heisenberg create the probabilistic interpretation (or Copenhagen interpretation) of quantum mechanics, where only observable quantities are used. Use it as a philosophy, and Heisenberg discards quantum mechanics entirely, because he didn't yet have the tools to see separate atoms, or electrons, or anything that quantum mechanics dealt with. But previous theories predicted those particles, and all experiments seemed to say that they existed, so they were used in further theories, and physics advanced.

Two guys named Thomson and Kaufmann do experiments on cathode ray tubes; after much bending of the cathode rays, they come up with the electric charge and mass of the ray. But Kaufmann, the positivist, who had a more accurate reading, reported it as the ray having a certain ratio of electric charge to mass. Thomson decides that he's discovered a new particle, even though he can't observe it; he begins more research, seeing the same charge and mass elsewhere, and soon, he's known as J. J. Thomson, the man who discovered the electron.

I guess the main point here is that there's gold to be panned from assumptions. And, besides, how can you say that what you 'observe' is, in fact, what is there? Kaufmann assumed the theories behind the working of the cathode ray were correct, and measured a visual spot on a screen; a true positivist would have only accepted that there was a bright spot on the screen in front of him, since that was all that could be directly measured by observation. Ceci n'est pas une node, indeed.

Seriously, visit Ceci n'est pas une node, if you didn't quite get the last part... I know it's unclear.

Proofread this! let me know if I got something wrong! I had to figure this out from scientific papers...

If you'll forgive me, I'm going to go drink observable quantities of alcohol now.

See also : Logical Positivism. I spend most of my noding hours treading over ground that my forebear, Pedro, has already treaded...