EVERY science has a subject-matter, which is investigated in that science. Thus, in the case of Medicine, it is human bodies from the standpoint of sickness or health, while for Geometry it is measurement. As for elementary principles, if they be not self-evident, they should be demonstrated in another science on a higher level than the science in question, and assumed67 in the latter. Thus, it is one of the principles of the Science of Medicine that the elements are four and four only, for this question has been demonstrated in Natural Science; the physician must therefore adopt it from the natural scientist, regarding it as assumed in his own science. Similarly, it is one of the principles of Geometry that there are continuous, constant measurements of only three sorts, linear, superficial and solid; for this fact is established in the Divine Science which is called Metaphysics, and it is for the geometrician to adopt it from the master of the latter science and apply it in his own. Metaphysics is that in which all sciences culminate, and it can have no elementary principles that are not self-evident; but there are questions investigated in that science, and in its very entirety it is restricted to those. An exposition of this preamble has been given exhaustively in the Science of Logic.