  It appears that all of the Arguments in D4 are valid. Like the Arguments of the first three Deductions, the Arguments of D4 rest on a number of independent premises, including premises of D1 and D2 (but not D3). Again, it appears that most of the premises are, at least from Plato’s point of view, either definitionally true or self-evident. There is one major exception to this claim, however. All of the Arguments of D4 depend for their soundness on the soundness of D1A2. But D1A2 depends for its soundness on the soundness of D1A1, which itself depends for its soundness on the truth of Purity-F.