Sentence:  ... most Writers are so far from instructing us in the Nature and Knowledge of Things, that they "use their Words loosly" and uncertainly, and do not, by using them constantly and steadily in the same signification, make plain and clear deductions of Words one from another, and make their Discourses coherent and clear, (how little soever it were instructive), which were not difficult to do, did they not find it convenient to shelter their Ignorance or Obstinacy, under the Obscurity and <WSD>perplexedness</WSD> of their Terms ... 
Meaning of the word marked with <WSD> tags:  The quality of being intricate, complicated or entangled.