Why does the Diamond Sutra say that a bodhisattva who gives without dwelling in marks has inconceivable merit?

Here, "marks" refers to the marks of objective realms — the six sense-object realms of form, sound, smell, taste, touch, and dharmas. When a bodhisattva practices generosity (dāna), he does not abide in the marks of form, sound, smell, taste, touch, or dharmas, nor does he cling to these marks; his mind empties these marks, and in giving, the three wheels are empty. He does not discriminate the recipient's gender or age, status or wealth, power or position; nor does he discriminate appearance, or whether the recipient is an enemy or a loved one — he gives equally to all. Nor does he calculate and ponder what benefits the giving might bring him from those people, nor expect returns in the form of the objective realms of form, sound, smell, taste, touch, and dharmas, or of wealth, sex, fame, food, and sleep; still less does he consider whether the value of the items given is high or low. As long as there is someone in need, he gives unconditionally, without calculation or discrimination.

One who can abandon all marks, cling to no marks, and give with the three wheels empty has a vast mind and is no longer an ordinary person — one whose mind has already been emptied is no longer an ordinary person; that is a great-minded bodhisattva in the true sense, one who possesses the profound wisdom of the emptiness of marks, a person of great merit. After such great giving, wisdom and merit advance another level. The merit of the wisdom of emptiness is not limited by quantity; the merit thus obtained is immeasurable and boundless, inconceivable. As vast as the mind is, so vast is the merit obtained; merit and capacity of mind are directly proportional and mutually reinforcing. But one who gives while clinging to marks has a mind constrained and obstructed by marks, so the capacity of mind is small and the merit obtained is small — how can a small vessel hold many things or large things? Merit follows the vessel: a great vessel brings great merit, a small vessel brings small merit — this is inevitable. Therefore, when a bodhisattva gives without abiding in marks, his capacity is great and his mind is like empty space, and the merit he obtains is like empty space — immeasurable and boundless, beyond reckoning.