About the concept of Chemical Space: a concerned reflection on some trends of modern scientific thought within theoretical chemical lore1.
Ramon Carbó-Dorca2

Sir! In a recent paper [1], a footnote was written by the present author as coauthor in order to comment about the concept of Chemical Space, which is a term appearing sporadically since old times in some available chemical literature. The mentioned footnote text was printed as follows, with the underlined part used to emphasize here some relevant parts:
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, …ver más…

The question mark is mine. Fortunately, this not a so reliable source of information, provided as a real bonus with a complementary reference to the subject [2]. Within this reference it was found the following text:
“Space”, as Douglas Adams famously said “is big. You just won't believe how vastly, hugely, mind-bogglingly big it is”. Change ‘space’ to ‘chemical space’, and his statement has similar resonance: the total number of possible small organic molecules that populate ‘chemical space’ has been estimated to exceed 10 - an amount so vast when compared to the number of such molecules we have made, or indeed could ever hope to make, that it might as well be infinite. So, it is not surprising that our exploration of chemical space has so far been extremely limited. Taking the analogy further, just as much of astronomical space is a void, much of chemical space contains nothing of biological interest. But rarely, and often through serendipity rather than design, we have identified ‘stars’ in chemical space — molecules that can modulate biological processes. These molecules have formed much of the basis of our fight against disease and have
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greatly aided our understanding of biological systems. But such successful finds have been hard to come by, in part because of our lack of understanding of chemical space. Given that its enormous size makes a thorough exploration of chemical space impossible,