About the concept of Chemical Space: a concerned reflection on some trends of modern scientific thought within theoretical chemical lore

Often in the QSPR or QSAR common literature the term Chemical Space appears. This concept it is not currently used with the same sense as a possible molecular space notion or the equivalent term: space of molecules given here, but in a generic fuzzy manner, sometimes even using meaningless and preposterous literary ways. The authors would like to find within the QSPR literature a sharp definition of such Chemical Space term, but until the present time found none. So, in order to employ the terminology of the present work as far away as possible from the usual classical QSAR fuzzy one employed by some authors, the term “space of molecules” has been chosen and will be used here, although “molecular space” might be also synonymou-sly used. This notion has been and will be also employed here as a companion of the term: space of parameters or the synonymous concept: space of descriptors, which is also a well-defined definition with a precise meaning, commonly used in the task to build up a set of discrete linearly independent vectors, empirically describing a MPC. From such a vectorial descriptor space