Something taken a priori is something that is taken without experience/evidence.
http://en.wikipedia.org/wiki/A_priori_and_a_posteriori#Use_of_the_terms
The scientific method uses a variety of assumptions that are taken to be useful because they help build a predictive model. There is no actual basis for these assumptions from evidence (and recursive evidence, that is taken from the results of the scientific method, does not count).
Hence I acknowledge that science can be predictively useful without actually producing a model that replicates or describes what is actually occuring in the real world (if indeed there is such a thing). Science's value is, and always has been, the ability to predict behaviour, if we can do that, then we can build on it, and that is the basis of all our progression. Science does not know or care if it has the 'correct' or 'complete' model, only if the model is useful, and that is both it's biggest advantage (when used correctly) and it's biggest flaw (when used incorrectly).
None of this devalues science in any way for the purpose for which it was concieved, it only addresses those who treat science like a new religious text, as something that contains
the answer to everything. the truth is that we don't know, and probably can never know, because we can never observe things in a mind and theory independant manner, and therefore we can never experience the truth, we can only work with our perception of it.