“My paintings are not about what is seen. 　　They are about what is known forever in the mind.” 　　– Agnes Martin 　　“'‘I’m very careful not to have ideas, because they’re inaccurate,’ the artist Agnes Martin says in Mary Lance’s touching documentary about her, but it’s a lie. Ms. Martin was full of ideas, and she dispenses them engagingly in the course of the film… She talks about her ...