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.”
“I know this medium of the Teaching, the ball of fragrance making offerings on meeting buddhas everywhere, pleasing all sentient beings; how can I know the practice or tell the virtues of the enlightening beings who are like great medicines, whom it is beneficial to see, to hear, to associate with, to remember, to follow, and to recall their names, with the sight of whom all sentient beings are relieved of afflictions, with the sight of whom sentient beings leave all evil ways, with the sight of whom sentient beings find a way to buddhahood, with the sight of whom the sufferings of sentient beings are relieved, with the sight of whom sentient beings are freed from fear of all mundane states, with the sight of whom sentient beings become fearless because of being near the abode of omniscience, with the sight of whom sentient beings no longer fall into the abyss of old age and death, with the sight of whom sentient beings attain the bliss of tranquillity through cosmic equanimity?
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.
“I know this enlightening liberation of the supernal manifestation of the sphere of unobstructed vows. How can I know the practice or tell the virtues of enlightening beings who are mines of inconceivable, immeasureable virtues, who have entered the indivisible ocean of buddha-bodies, who take in the indivisible multitude of truths, who have attained an indivisible ocean of virtues, who have spread the net of the practice of universal good, who have entered the realm of indivisible absorption, whose roots of goodness are one with those of all enlightening beings, who abide in the pure nondiscrimination of buddhas, who have entered the indivisible equality of past, present, and future, who live continuously through all ages without tiring, who are in the stage of the sphere of the unfragmented universal eye? “