the most perfect solitude
If the reader regards this phrase ironically, it would seem commensurate (or nearly so) with death. The last third of Frankenstein is dominated by the withdrawal of its protagonists behind psychic barricades that wall them off increasingly from the forces of life. Again, one senses the context of P.B. Shelley's recently-published Alastor; or, The Spirit of Solitude behind Mary Shelley's text.