Our knowledge is doubling every 100 years: Laying the foundation for the „measuring of knowledge”︁. The slogan of the doubling of our knowledge every 15 years is well‐known. But there are three methods leading consistently to a doubling period of 100 years (the 2nd and the 3rd method show, moreover, that modern science began in the 2nd half of the 15th century, and that there were several downs, with a particularly low one about 1705): The comparison of text books (of (...) different age), the growing number of important discoveries per period of time (selected by authors of chronological tables) and the growing number of famous scientists (included by encyclopedias or mentioned in works on the history of science). (Books which are suitable as a basis for such research are compiled in a comprehensive bibliography.) The counting of research objects like chemical elements, animal and plant species, stars etc. shows very different results and is not very appropriate for measuring the speed of growth of knowledge; quite inappropriate for this purpose is measuring the number of all research workers and the whole scientific literature. We must make a distinction whether we want to measure the growth of science or that knowledge. Previous attempts to the „measuring of knowledge”︁ are shortly summarized. (shrink)