佛祖統紀 3.132

3.132

祕密得名。玄云。顯露不定。祕密不定。籤云。為對祕密須安顯露（云云）由前四味有一類機。顯露不解宜祕密入。是故如來不思議力。隱覆密說。要令眾生互不相知得漸頓益耳。

The derivation of the name 'Secret': The [Miao]xuan says: 'Manifest indeterminate and secret indeterminate.' The Qian says: 'To contrast with the secret, one must posit the manifest' (and so on). Because among the prior four flavors there is a class of capacities that does not understand through the manifest and is suited to enter via the secret, therefore, through the Tathāgata's inconceivable power, he conceals and secretly expounds it, causing sentient beings to be mutually unaware and yet obtain the gradual or sudden benefits.