One of the most striking features of the philosophy of the middle and later Wittgenstein is his decision, exemplified in the above quotation, to assimilate necessary truths to arbitrary rules of grammar. These truths include not only analytic ones, but also any considered to be synthetic a priori, such as truths which might be regarded as, in a broad sense, having to do with categorial propriety. Under the heading of truths of categorial propriety we may further distinguish two subclasses.