Hence the reality of Mathematical Knowledge I doubt not but it will be easily granted, that the knowledge we have of mathematical truths is not only certain, but real knowledge; and not the bare empty vision of vain, insignificant chimeras of the brain: and yet, if we will consider, we shall find that it is only of our own ideas.
The mathematician considers the truth and properties belonging to a rectangle or circle only as they are in idea in his own mind.

For it is possible he never found either of them existing mathematically, i.e.
precisely true, in his life.

But yet the knowledge he has of any truths or properties belonging to a circle, or any other mathematical figure, are nevertheless true and certain, even of real things existing: because real things are no further concerned, nor intended to be meant by any such propositions, than as things really agree to those archetypes in his mind.

Is it true of the IDEA of a triangle, that its three angles are equal to two right ones?
It is true also of a triangle, wherever it REALLY EXISTS.

Whatever other figure exists, that it is not exactly answerable to that idea of a triangle in his mind, is not at all concerned in that proposition.