Sarvasya means everything, the Supersoul. Sarvasya ca aham Kṛṣṇa. Kṛṣṇa says: "I," hṛdi, "in the heart." When it is called hṛdi, that means living entity. Without living entity, there cannot be any heart. Without this body, no, there cannot be any heart. In the stone there cannot be any heart. Dead body. The heart is dead.
So sarvasya ca ahaṁ hṛdi. Hṛdi means "in the heart." Sanniviṣṭaḥ: "I am situated. I am sitting there." And mattaḥ smṛtir jñānam apohanaṁ ca (BG 15.15).
Because He is sitting there, therefore all kinds of remembrance, smṛti, knowledge and forgetfulness. These things are functions of the mind. You remember something, you gather some knowledge, and sometimes you forget. "Let me think. Oh, yes. I forgot, yes." That's a function of the mind.
So that forgetfulness is also due to Kṛṣṇa, because He's sitting there. He's guiding. If you want to forget Kṛṣṇa, He'll give you so opportunity of māyā that you can . . . you'll never remember Kṛṣṇa. You'll forget for good. And that is His kindness. "You want to forget Me? All right. I give you so much facility that you'll never be able to remember Me." Therefore the staunch atheist, however you argue with him, however you defeat him . . . matir na kṛṣṇe parato svato vā (SB 7.5.30).
They cannot. They will not. Because Kṛṣṇa is arranging within, "Forget. Don't accept it. Don't accept it." So this is going on. Sarvasya cāhaṁ hṛdi sanniviṣṭaḥ (BG 15.15).
So for the nondevotees, He's giving facilities to forget. And for the devotees, who are sincerely seeking Kṛṣṇa's favor, Kṛṣṇa's service, He's giving memorization, "Yes, you do like this. Do like this." Why does He so? That is also explained, ye yathā māṁ prapadyante (BG 4.11)
"As you want . . . if you want Me, to forget, I give you facility to forget.