Kamalasila’s second Stages of Meditation says:
With bare insight that lacks serenity, the yogi’s mind is distracted by objects; like an oil-lamp in the wind, it will not be stable. For this reason, what sublime wisdom sees will not be very clear. As this is so, rely equally on both.
As the Moon Lamp Sutra says:
The power of meditative serenity makes your mind steady; insight makes it like a mountain.
So, the mark of meditative serenity is that your attention stays right where it is placed without distraction from the object of meditation. The mark of insight is that you know the reality of selflessness and eliminate bad views such as the view of self; your mind is like a mountain in that it cannot be shaken by opponents. Therefore, you should distinguish these two marks.
Kamalasila’s first Stages of Meditation says:
Because your mind moves like a river, it does not rest without the foundation of meditative serenity; a mind that is not in meditative equipoise cannot understand reality just as it is. Also, the Bhagavan says, “With meditative equipoise, you know reality just as it is.”
The technique for producing forceful and longlasting certainty about the meaning of selflessness is sustained analysis with discerning wisdom. Without such insight into the real nature, no matter how long you cultivate serenity, you can only suppress manifest afflictions; you cannot eradicate their seeds. Therefore, do not cultivate only serenity; you need to cultivate insight as well
Also, the Scriptural Collection of the Bodhisattvas says:
Those who are unlearned in the contents of the Scriptural Collection of the Bodhisattvas, unlearned in the discipline of the noble teaching, and who derive a sense of sufficiency from mere concentration (三摩地) fall by virtue of their pride into an inflated sense of themselves. They will not escape from birth, aging, sickness, death, sorrow, lamentation, suffering, unhappiness, or perturbation; they will not escape from the six realms of cyclic existence; they will not escape from the aggregation of suffering. With that in mind, the Tathagata said, “Learning from others what is appropriate, you will escape aging and death.”
On this point, the Ratna-kuta Collection says:
Keeping ethical discipline, you will attain concentration;
Attaining concentration, you cultivate wisdom;
With wisdom you attain pure, sublime wisdom;
As your sublime wisdom is pure, your ethical discipline is perfect.
According to the Sutra Unravelling the Intended Meaning, as long as the practice of discrimination and special discrimination with discerning wisdom cannot generate physical and mental pliancy, it constitutes a type of attention which approximates insight; when it generates pliancy, then it is insight.
So generating pliancy by setting your attention on a single object of meditation – even if the object is emptiness – is nothing more than a way to achieve serenity (止); that alone does not count as attaining insight.
Accordingly, the way insight develops is that discerning analytical meditation generates pliancy. If this were not so, there would not be the slightest good reason to seek serenity first and then cultivate insight based on it.
This means that they have not attained the serenity of the actual first meditative stabilization, or beyond; it does not preclude their having attained the serenity which is included in the access to the first meditative stabilization.
For Asanga’s Levels of Yogic Deeds says:
Moreover, you can accurately know the reality of the truths from suffering to path, without having attained the first meditative stabilization, etc. As soon as this knowledge of the truths occurs, you stabilize your mind and do not analyze phenomena. Based on this higher wisdom, you pursue the practice of higher states of consciousness.
The area should have five attributes: (a) easy access, so that necessities such as food and clothing may be readily obtained; (b) being a good place to live, where there are no wild beasts such as predators, no enemies, etc.; (c) being on a good piece of ground, in that it does not breed sickness; (d) offering good companionship insofar as your companions are ethically disciplined and likeminded; and (e) being well-situated inasmuch as there are not many people about in the day and little noise at night.
Maitreya’s Ornament for the Mahayana Sutras states:
The intelligent practice in a place
Which is accessible, is a good place to live,
Offers good ground and good companions,
And has the requisites for comfortable yogic practice.
(5′) Pure ethical discipline
You do not violate precepts, doing deeds that are wrong by nature or wrong by prohibition, either in the case of vows of individual liberation or in the case of bodhisattva vows. If you do violate them through carelessness, you restore them promptly with regret in accordance with the teaching.