The Four Zens of the Lankavatara Sutra
To practice dhyana, the earnest disciple should retire to a quiet and solitary place, remembering that life-long habits of discriminative thinking cannot be broken off easily nor quickly. There are four kinds of concentrative meditation (Zen, dhyana): The dhyana practised by the ignorant; the dhyana devoted to the examination of meaning; the dhyana with "suchness" (tathata) for its object; and the dhyana of the Tathagatas.
The dhyana practised by the ignorant is the one resorted to by those who are following the example of the disciples and masters but who do not understand its purpose and, therefore, it becomes "still-sitting" with vacant minds. This dhyana is practised, also, by those who, despising the body, see it as a shadow and a skeleton full of suffering and impurity, and yet who cling to the notion of an ego, seek to attain emancipation by the mere cessation of thought.
The dhyana devoted to the examination of meaning, is the one practised by those who, perceiving the untenability of such ideas as self, other and both, which are held by the philosophers, and who have passed beyond the twofold-egolessness, devote dhyana to an examination of the significance of egolessness and the differentiations of the Bodhisattva stages.
The dhyana with Tathata, or "Suchness," or Oneness, or the Divine Name, for its object is practised by those earnest disciples and masters who, while fully recognising the twofold egolessness and the imagelessness of Tathata, yet cling to the notion of an ultimate Tathata.
The dhyana of the Tathagatas is the dhyana of those who are entering upon the stage of Tathagatahood and who, abiding in the triple bliss which characterises the self-realisation of Noble Wisdom, are devoting themselves for the sake of all beings to the accomplishment of incomprehensible works for their emancipation. This is the pure dhyana of the Tathagatas. When all lesser things and ideas are transcended and forgotten, and there remains only a perfect state of imagelessness where Tathagata and Tathata are merged into perfect Oneness, then the Buddhas will come together from all their Buddha-lands and with shining hands resting on his forehead will welcome a new Tathagata.
-The Lankavatara Sutra