Long story short, the -osis suffix led me to <diagnosis>, which brought me to <diagnose> and <diagnostic>, which brought me to <Gn> “know.” 

Di + a + Gn + ose -> diagnose

Di + a + Gn + ose + is -> diagnosis

What do we do with <diagnostic>? Is -ost a variation of -ose? Or can we not go down to <Gn> as the base? Do we have <Gnose> and <Gnost> instead?

Then we get to <cognition>. Do we have <co + Gn + ite + ion> or can we go down only to <Gnite>?