  Before offering an explanation of the generation of the world, Timaeus sets out the fundamental principles that will govern his whole explanation of the physical world (Tim. 27d5–28b5). As Proclus observes, it is the task of a scientist to formulate at the start of his project the principles proper to the science in question, and not just to assume some general axioms. The science of nature too is based on specific axioms and assumptions, which must be clarified before we can move to the demonstration. In order to make phusiologia a real science, the philosopher must deduce his explanation, as does the geometer, from a set of fundamental propositions or axioms.