He based his argumentat on the document from 26 April 1177, where Leszek was mentioned as taking the last place among Polish princes, after Mieszko younger (Misico iunior dux).
Dia mendasarkan argumennya pada dokumen tersebut dari tanggal 26 April 1177, di mana Leszek disebut-sebut mengambil tempat terakhir di antara para pangeran Polandia, setelah Mieszko muda (Otot iunior dux).