When he was 18, he studied under Eiku at Kurodani Valley in the Saito precinct deep within Mt. Hiei-zan, where he took one character from the names of each of his teachers Genko and Eiku to create the name Honenbo Genku.
18歳で比叡山でも奥深い山中にある西塔黒谷の叡空に師事し、源光と叡空の名前の1字ずつを取って法然坊源空と改名した。