In the third phase, the nineteenth-century discovery of the conservation of energy continued to allow that sui generis non-physical forces can interact with the physical world, but required that they be governed by strict force laws. Sui generis mental and vital forces were still widely accepted, but an extensive philosophical debate about the significance of the conservation of energy led to a widespread recognition that any such forces would need to be law-governed and thus amenable to scientific investigation. We might usefully view this as a species of ontological naturalism that falls short of full physicalism.  Mental and other special forces were still sui generis and non-physical, but even so they fell within the realm of scientific law and so could not operate spontaneously. (As many commentators at the time recognized, this weaker form of naturalism already carried significant philosophical implications, particularly for the possibility of free  will.[5]) 