The sun does not come up in the morning and go down at night, it just ‘appears’ to do this because the earth is rotating but the sun remains still.
A good putting stroke ‘appears’ to follow the path of an arc through the impact zone.
In fact, a good putting stroke path is when the putter head stays in a plane. Imagine the plane being a disc of glass which passes through the spine at a point between the shoulder blades and the putter head. As long as the putter heads stays in the plane — and the golfer does not rotate the wrists — then the putter head will remain square to the plane throughout the stroke.
However, because the golfer’s head and eyes are not in the plane but through it (see sketch below) the path will ‘appear’ to be in an arc.