One who believes there is no supernatural, merely the preternatural.
In other words, someone who believes that for any phenomena that appear supernatural (ie: miracles), there exists a yet unknown, rational explanation that is simply beyond the scope of current knowledge.
Christian circa 1st century C.E.: Rainbows are a miracle from God and His promise to not flood the Earth again.
Preternaturalist circa 1st century C.E.: I don't know how rainbows are formed, but I believe there is a rational explanation for their existence.
Sir Isaac Newton circa late 17th century C.E.: Rainbows occur due the refraction of light. Check this sweet ass rainbow I just made with a prism.