I have a problem with the following sentence. I am not sure whether I could grasp the intended meaning or not. Could you please reformulate the sentence, especially the part in bold-face in order for it to make more sense?
Many thanks in advance.
"The a priori method consists of demonstrating the necessary agreement or disagreement of anything with a rational and social nature, whereas the a posteriori method follows the more fallible course of concluding, if not with absolute assurance, at least with every probability, that that is according to the law of nature which is believed to be such among all nations, or among all those that are more advanced in civilization."