Fourthly, Of real Existence agreeing to any idea.
FOURTHLY, The fourth and last sort is that of ACTUAL REAL EXISTENCE agreeing to any idea.
Within these four sorts of agreement or disagreement is, I suppose, contained all the knowledge we have, or are capable of.

For all the inquiries we can make concerning any of our ideas, all that we know or can affirm concerning any of them, is, That it is, or is not, the same with some other; that it does or does not always co-exist with some other idea in the same subject; that it has this or that relation with some other idea; or that it has a real existence without the mind.

Thus, 'blue is not yellow,' is of identity.

'Two triangles upon equal bases between two parallels are equal,' is of relation.

'Iron is susceptible of magnetical impressions,' is of co-existence.