How Useful Is Economics?

My professor insists that it is possible to determine the shift in a demand curve given some change in technology, supply or other variable.

I overstated my case stating that these were guesses (highly educated guesses, I would have been better off saying estimates) and that it isn't possible to 'know' what the new demand curve would be until you could observe it.

What are your thoughts? Is it possible to know what a demand curve will be if you change certain determinants?

Tags: economics, school