What are the motifs in The Secret Diary of Laura Palmer by Jennifer Lynch?

The physical versus the spiritual worlds are recurring idea. The manifestations of a third, that of the tensions and differences between the worlds of physical and spiritual experience. This dynamic is particularly interesting, in that each is portrayed as both good and bad, with physical experience (i.e., sexuality) being portrayed as simultaneously dangerous and empowering at the same time as spiritual experience, such as in the form of the evil BOB and the enlightening Log Lady, being portrayed as simultaneously terrifying and, again, empowering. It's important to note, meanwhile, that in keeping with the book's concept of duality as outlined above, there is little or no sense that neither the physical nor the spiritual realities are intended to be considered as purely "bad" or purely "good".