Kant on things in themselves

Everyone is free to debate about this, but I won't discuss it with
Michael Gordge if he chooses to use personal attacks.

Kant gives us an example in an attempt to illustrate how we cannot
know things in themselves.

>From Critique of Pure Reason (A 48/B 65):

"Take the proposition that three straight lines permit construction of
a figure, and try ... to derive it from these mere concepts ... Now
suppose that there did not lie within you a power to intuit a
priori ... and that the object (the triangle) were something in
itself ... If that were so, how could you say that what necessarily
lies in [or belongs to] your subjective conditions for constructing a
triangle must also belong necessarily to the triangle itself? For,
after all, you could not add to your concepts (of three lines)
anything new (the figure) that would therefore have to be met with
necessarily in the object, since this object would be given prior to
your cognition rather than through it. Hence you could not
synthetically a priori establish anything whatsoever about external
objects ..."

The mathematical form of the statement is,

(El)(Em)(En)(Line(l) & Line(m) & Line(n) & EnclosesATriangle(l, m,
n)),

stated in Euclidean geometry, where (Ex) means, "there exists an x
such that ..."

First of all, I consider three lines to be objects, not concepts.
Second, it's not clear to me what he means when he says that "this
object would be given prior to my cognition rather than through it."