Do scientists assume their conclusions?

A brief exchange I had on Facebook. In context, McRae is responding to a young-earth creationist:

Steve McRae 
A real scientist doesn't assume their conclusion, they go where the evidence leads them. No scientist should EVER start with a conclusion. That is just bias and not how science is done.

Steve Hays
Wasn't Relativity inspired by thought-experiments and mental pictures long before Einstein had empirical confirmation? What about Pauli's dreams. Or Dirac's mathematical intuition, based on "beauty"? What about Newton's bucket and Newton's canon?

Actually, a basic function of scientific theorizing is to go beyond the available evidence by making predictions. In many cases, a scientist wouldn't need to make a prediction in the first place if he already had the evidence in hand. Predictions are not simply ways of testing a theory, but discovering new evidence. A theoretical prediction points scientists in a particular direction. They look for evidence where the theory predicts they should find it. Sometimes that confirms the theory, sometimes that discomforts the theory. 

Take Bell's theorem. That was formulated well before the equipment existed to test the theoretical experiment. 

McRae is operating from a simple-minded positivism.