Theme or Plot

A writing group that I'm a part of had an interesting and long running discussion on writing . . . is it plot driven or theme driven. 

When I write, and I've published 3 novels (see HERE), I start with plot and from that develop characters and theme. 

Plot is always the driving factor with me. In fact in all three cases I've developed themes both major and minor as I have written. One or two times the primary theme was already there in my mind, and then as I wrote the minor theme exposed themselves and turned into something more substantial. 

I really like the fact that these themes have provided greater depth to my writing and my story by the themes further development. Unlike the plot, which is the crux of the story for me, the themes always start springing up naturally as I write.