Sentence:  To avoid, therefore, all imputation of laying down a rule for posterity, founded only on the authority of <WSD>"ipse dixit</WSD>"—for which, to say the truth, we have not the profoundest veneration... 
Meaning of the word marked with <WSD> tags:  A dogmatic and unproved proposition or dictum that is accepted solely on the authority of someone who is known to have asserted it.