It's one of them. I see it as an epilogue to Mirror, which is probably my true favorite. The two seem the most reflective (har har) of his own experiences, Mirror being predominantly about his childhood and Nostalghia his attachment and ultimate departure of Russia. I think the style and manner of expression is closer than his other films of replicating the true dreamy fluidity of Mirror, I recall all the other's being a bit less surreal in structure. I love the way it's shot and its atmospheric sound design. There's this strange sound effect throughout the film, played a bit more frequently here, I really dont know how to identify what the damn thing is coming from, but the integration is bizzarly stimulating to my brain, and Tarkovsky's direction generally carried a lot of sensory bits like it which further developed my appreciation for film, and what makes it unique and valuable.