For us, the FTC is about a relation between a continuous function f(x) and its indefinite integral F(x) = òxx0 f(t) dt, so it may be surprising that Gregory was able to state the result without using the language of functions. The idea behind the alternative geometric approach is simply to focus on curves like those formed by the graphs of f(x) and F(x). In particular, instead of considering a function f(x) graphed on the x- and y-axes, suppose that OA and OB are line segments perpendicular to each other at the point O and that OFL is a curve that `passes the vertical line test', so that it can be thought of as formed by the graph of a function f(x). (Following the custom of many classical Euclidean geometers, Gregory denotes an arbitrary curve by three points chosen at random on the curve. Later on, these points will each take on a specific meaning.) Next, define another curve OEK point-wise as follows: at each point I on the line segment OA the length IK of the perpendicular line segment between OA and OEK is equal to the area of the curved region OFLI.
In modern terms, the curve OEK is the graph of F(x) = òx0f(t) dt. The FTC states that F¢(x) = f(x) for each x in the interval. Since F¢(x) is the slope of the tangent line to F(x) at x, in geometrical terms this is equivalent to saying that for any I on the line segment OA, if C is the point where the tangent line KC to the curve OEK at K intersects OA, then the slope IK/IC of this tangent line equals IL. The proof will be established by showing that this relation holds between the points C, K, and L.