Tanasije has recently offered up Einstein as an example of how empirical science is dependent on a priori knowledge. His point seems to be that once Einstein had his two fundamental principles (i.e. the principle of relativity and the principle of the constancy of the speed of light) he was able to use ‘pure reason’ and deduce a priori the rest of his physics. But the question of a priori knowledge, I think, is the question of the status of the first principles and the status of the rules that we use to deduce the rest of the physics. It is only if you think that the two principles + rules for deduction are known to be necessary facts about the nature of reality independently of experience and solely by reason.
The empiricist can account for what Einstein did; the two principles followed from empirically validated theories of the time (the principle of relativity is stated by Galileo and is a part of Newtonian physics as Einstein knew it and the constancy of light is predicted by Maxwell’s equations). The rules that are used to deduce (i.e. classical logic) are highly useful empirical generalizations. So Einstein does indeed start from first principles and deduce physics (if that is indeed what happened), but nothing in the story told by the empiricist is really independent of experience.
In order to have some serious rationalism going on, one would have to add to Tanasije’s accoount the claim that Einstein’s principles and the rules of classical logic are known by reason to be necessary facts about reality. Einstein does seem to cite a thought experiment as evidence for the constancy of the speed of light. He says he imagines chasing after a beam of light. What would it look like? If Einstein could catch up with the beam and look at it how would see the light standing still, which is unintuitively odd. So, he must not actually be able to catch up to the beam. But the only way that that is possible is if its speed were constant relative to Einstein no matter how fast he went. QED.
But is this really evidence for rationalism? Not quite. Some scientists (Paul Davies, for instance) think that that the speed of light may have been slowing down since the Big Bang. What is going on here seems to be this. We have a theory wich is the one that best unites disperate phenomena and is empirically adequate. We usually have outlying data and often scientists take creative leaps to integrate these outlying data points and thereby unite more disperate phenomena and provide greater empirical adequacy. It is plausible to think that Einstein himself was motivated by a conflict between theories he found intuitively compelling for the reasons cited above.
The moral of the story? Intuitions are theory driven and not ‘tother way ’round!