The infinitive limiting the meaning of an adjective is commonly active (or middle) in cases where the passive is more natural in English. Thus,
λόγος δυνατὸς κατανοῆσαι
a. The active use is due to the old datival function of the infinitive: δυνατὸς κατανοῆσαι
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].