This is fine for the leptons and the up and down quarks, but leaves the strange quark in isolation and out of the weak interaction. Even looking at this, it seems asymmetric, almost demanding that there be another doublet of quarks to fill the empty space in the lower-left corner. In his original paper postulating quarks, Gell-Mann alluded to the possibility of a fourth quark (Figure 9.1) that formed a pair with the strange quark, ‘by analogy with the leptons.’ This idea was briefly pursued by Glashow, among others, but was then dropped because not a single hadron containing a ‘charmed’ quark was found.