Inquiry into our conception of Power or Causation becomes prominent in Reid’s letters to Lord Kames throughout ‘the seventies,’ along with experimental investigations in physics and physiology which show continued interest in natural causes. A letter, written in 1775, contains a curious conjecture with regard to the generation of plants and animals, more speculative than was his habit. He is ‘apt to conjecture,’ he tells his friend, ‘that both plants and animals are at first organised atoms, having all the parts of the animal or plant, but so slender, and folded up in such a manner, as to be reduced to a particle far beyond the reach of our senses, and perhaps as small as the constituent parts of water. The earth, the water, and the air may, for anything I know, be full of such organised atoms.’ He then goes on to consider the relation of this hypothesis to the idea of active design in nature, and expresses doubt about the possibility of the atoms being endowed with power to form themselves into an organised body like the human. ‘I cannot help thinking that such a work as the Iliad, and much more an animal or vegetable body, must have been made by express design. It seems to me as easy to contrive a machine which should compose a variety of epic poems and tragedies, as to contrive laws of motion by which unthinking particles of matter should coalesce into a variety of organised bodies.’ He suggests that the organisation is the issue of constant and uniform divine activity. ‘Can we,’ he asks, ‘show by any good reason that the Almighty finished his work at a stroke, and has continued ever since an inactive spectator? And if His continued operation be necessary, it is no miracle, while it is uniform, and according to fixed laws. Though we should suppose the gravitation of matter to be the immediate operation of the Deity, it would be no miracle while it is constant and uniform; but if it should cease for a moment, only by His withholding His hand, _this_ would be a miracle.’ This is to say that all natural changes are immediate effects of divine action, proceeding according to natural law or rule.