In additional to their professional achievements, scientists often have a "natural philosophy" to guide them in their choice of problems and theory construction. To the degree that this natural philosophy raises general conceptual questions, often outside the range of science to decide on them empirically, it is hardly possible to separate philosophical and scientific work Unlike a metaphysical system, however, a natural philosophy may be only a working picture of the world standing opposite the scientist's physics while, like metaphysics, it may not be subject to direct experimental test. A natural philosophy may only touch physics at a high level of generality or may represent an ideal point to which the most general physical principles converge. In Mach's time, the mechanical worldview of particles and action and a distance, the electromagnetic worldview of fields and action by contact, and the thermodynamic view of pure energy transformations without regard to mechanism were all natural philosophies that strove with one another for the mastery.