  Since all ideas originate in sensation, Regius had to acknowledge that we often come to have ideas that originate from imagined realities and that, in the case of veridical and non-veridical perceptions, the subjective experiences are often sufficiently similar that one cannot be certain of a correspondence between ideas and reality. In contrast with the Cartesian rational defence of the reliability of our cognitive faculties, Regius claimed that one must rely on Scripture for this purpose (Regius 1646, 249). However, Scripture does not guarantee that our perceptions are always certain. This issue was particularly acute when the realities in question were unobservable (Regius 1646, 4).  Regius thought it was obvious that physical bodies are composed of insensible particles, and that their description is a matter of hypothesis. When faced with the task of explanation, therefore, he took refuge in some of the central themes of Cartesian natural philosophy. He borrowed the ‘fable’ used by Descartes in Le Monde, that the natural world is composed of one matter, that it is divided into parts of various sizes and shapes in motion, and that all natural phenomena may be explained by the laws of nature that apply to those parts (Regius 1646, 4).