and therefore we cannot with certainty affirm:  That all men sleep by intervals; That no man can be nourished by wood or stones; That all men will be poisoned by hemlock:  because these ideas have no connexion nor repugnancy with this our nominal essence of man, with this abstract idea that name stands for.  We must, in these and the like, appeal to trial in particular subjects, which can reach but a little way.  We must content ourselves with probability in the rest:  but can have no general certainty, whilst our specific idea of man contains not that real constitution which is the root wherein all his inseparable qualities are united, and from whence they flow.  Whilst our idea the word man stands for is only an imperfect collection of some sensible qualities and powers in him, there is no discernible connexion or repugnance between our specific idea, and the operation of either the parts of hemlock or stones upon his constitution.  There are animals that safely eat hemlock, and others that are nourished by wood and stones:  but as long as we want ideas of those real constitutions of different sorts of animals whereon these and the like qualities and powers depend, we must not hope to reach certainty in universal propositions concerning them.  Those few ideas only which have a discernible connexion with our nominal essence, or any part of it, can afford us such propositions.  But these are so few, and of so little moment, that we may justly look on our certain general knowledge of substances as almost none at all.

16.  Wherein lies the general Certainty of Propositions.

To conclude:  general propositions, of what kind soever, are then only capable of certainty, when the terms used in them stand for such ideas, whose agreement or disagreement, as there expressed, is capable to be discovered by us.  And we are then certain of their truth or falsehood, when we perceive the ideas the terms stand for to agree or not agree, according as they are affirmed or denied one of another.  Whence we may take notice, that general certainty is never to be found but in our ideas.  Whenever we go to seek it elsewhere, in experiment or observations without us, our knowledge goes not beyond particulars.  It is the contemplation of our own abstract ideas that alone is able to afford us general knowledge.

CHAPTER VII.  OF MAXIMS

1.  Maxims or Axioms are Self-evident Propositions.

There are a sort of propositions, which, under the name of maxims and axioms, have passed for principles of science:  and because they are self-evident, have been supposed innate, without that anybody (that I know) ever went about to show the reason and foundation of their clearness or cogency.  It may, however, be worth while to inquire into the reason of their evidence, and see whether it be peculiar to them alone; and also to examine how far they influence and govern our other knowledge.