18. Thirdly, Of relations between abstracted ideas it is not easy to say how far our knowledge extends.
Thirdly, As to the
third sort of our knowledge, viz., the agreement or disagreement of any of our ideas in any other relation: this, as
it is the largest field of our knowledge, so it is hard to determine how far it may extend: because the advances that
are made in this part of knowledge, depending on our sagacity in finding intermediate ideas, that may show the
relations and habitudes of ideas whose co-existence is not considered, it is a hard matter to tell when we are at an
end of such discoveries; and when reason has all the helps it is capable of, for the finding of proofs or examining
the agreement or disagreement of remote ideas. They that are ignorant of Algebra cannot imagine the wonders in
this kind are to be done by it: and what further improvements and helps advantageous to other parts of knowledge
the sagacious mind of man may yet find out, it is not easy to determine. This at least I believe, that the ideas of
quantity are not those alone that are capable of demonstration and knowledge; and that other, and perhaps more
useful, parts of contemplation, would afford us certainty, if vices, passions, and domineering interest did not
oppose or menace such endeavours.
