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.
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.
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.”
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
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.”
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 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.
Therefore, since bliss, clarity, and non-discursiveness are present in concentrations that know emptiness, yet very often occur in concentrations that are not directed toward emptiness, you have to differentiate these two.