Complex sentence: Bozal has suggested that those paintings also were painted by Goya, as this is the only way to understand why he reused them, however, Nigel Glendinning assumes that the paintings "already adorned the walls of Quinta del Sordo when he bought it".
Split into simpler sentences: Bozal has suggested that those paintings also were painted by Goya as this is the only way to understand why he reused them. However, Nigel Glendinning assumes that the paintings "already adorned the walls of Quinta del Sordo when he bought it".