The science of astronomy was begun by amateur and today remains dependent on their contributions, which are incisive by virtue of being _____ by the a priori assumptions that often vitiate the work of professional research scientist.
which are incisive 修饰their contribution, by virtue of .... the a priori assumption 是解释incisive, 那么that often 后面的从句是修饰的啥?thir contribution 还是a priori assumption?