Invention indeed is the hardest part, when Proofs are found it is not very difficult to manage them. And to know precisely wherein their Nature consists, may help us somewhat in our enquiries after ’em. An Intermediate Idea then which can make out an Agreement between other Ideas, must be Equivalent to, and yet Distinct from those we compare by it. Where Ideas agree it will not be hard to find such an Equivalent, and if after diligent search we cannot meet with any, ’tis a pretty sure Sign that they do not agree. It is not necessary indeed that our Middle Idea be Equivalent in all respects, ’tis enough if it be in such as make the Comparison: And when it is so to one of the Compar’d Ideas but not to the other, that’s a Proof that they do not agree amongst themselves. All the Commerce and Intercourse of the World is manag’d by Equivalents, Conversation as well as Traffick. Why do we Trust our Friends but because their Truth and Honesty appears to us Equivalent to the Confidence we repose in ’em? Why do we perform Good Offices to others, but because there’s a proportion between them and the Merit of the Person, or our own Circumstances? And as the way to know the Worth of things is to Compare them one with another, so in like manner we come to the Knowlege of the Truth of ’em by an Equal Ballancing. But you will say, Tho I may learn the value of a _Spanish_ Coin by Weighing, or Comparing it with some other Money whose Standard I know, and so discern what proportion it bears to those Goods I wou’d exchange; yet what Scales shall I find to weigh Ideas? What Hand so even as to poize them Justly? Or if that might be done, yet where shall I meet with an Equivalent Idea when I have occasion to use one?