A circular argument is logically flawed - it commences from the premise that what is to be proved is true. There's no such problem with the circle in art, however. It has long been considered a powerful symbol of simplicity, perfection, continuity and eternity.
A simple, but perfect, geometric shape, the circle can be found everywhere. From the humble donut to stunning modern architectural creations, circles lend themselves to interesting images.
Tech Talk
These images were taken over a couple of months, so the equipment varies. 1 and 3 were Olympus E-30 and Zuiko Digital 14-54mm f2.8-3.5 during one of our lunchtime photo walks, appropriately themed "circles". No. 2 was Olympus E-30 and Zuiko Digital 50-200mm f2.8-3.5. No 4 was Olympus E-5 and Zuiko Digital 7-14mm f4.
The 7-14mm is an amazing lens. For such a wide field of view (14-28mm full frame equivalent), it provides an extremely sharp and well corrected rectilinear image. Arguably the finest ultra-wide angle lens available on any camera system, and one of my favorite photographic tools.