A doctrinal classification system as explained by Zhiyi. The order in which S/a^kyamuni taught the dharma is broken down into five periods （see wu-chiao 五教）. The methods which he used are also broken down into four types， and the contents of the teaching into four categories. （A） The four methods of teaching are: （1） the Sudden Teaching 顿教， which is the teaching of the direct awakening to the Buddha-nature; （2） the Gradual Teaching 渐教， where one is lead from shallow understanding to deep understanding; （3） the Secret Indeterminate Teaching， where the aspirants secretly receive teaching appropriate to their own abilities without being aware of the presence of others and （4） the Express Indeterminate Teaching， where the listeners， though aware of each other， still receive teaching according to their individual capacities. There is， in addition to this， the method of "neither sudden nor gradual， neither secret nor variable"， also known as the "method of the Lotus and Nirva^na Sutras. （B） The four types of content are （1） Hi^naya^na or "Tripitaka" teachings; （2） "Pervasive" or "Shared" 通teachings. This is Maha^ya^na teaching that includes the teaching of Vaipulya， Prajn~a^pa^ramita^， Lotus and Nirva^na， and is for /sra^vakas， pratyekabuddhas and bodhisattvas; （3） the Distinct Teaching. This is teaching for bodhisattvas only. The practitioners understanding has its emphasis on the middle truth of understanding emptiness and existence. Huayan teaching is typical of this type. （4） The Perfect Teaching. Enlightenment and delusion do not differ in essence. The teaching of the perfect mutual penetration and containment of all things， the explanation of the Buddha's enlightenment exactly as it is. The perfect teaching is actually contained in all the other teachings， but is most excellently put forth in the Lotus Sutra 法华经.