Leviticus 18:10

The nakedness of thy son's daughter, or of thy daughter's daughter ,.... A man might not marry his granddaughter, whether a descendant of his son or of his daughter, nor any further off descending from him in a right line, not his great-granddaughter, and so on; and if he might not marry his granddaughter, much less his own daughter, as Jarchi observes, for the relation is still nearer; therefore that being prohibited, this in course must, though not mentioned:

even their nakedness thou shalt not uncover ; neither debauch nor marry such an one:

for theirs is thine own nakedness ; which sprung from his, being the descendants either of his son or daughter; the Targum of Jonathan is,"for they are as thy nakedness,'his own flesh and blood.