Because Buffett knows that Graham was right, he doesn't worry when stocks in Berkshire Hathaway's (NYSE: BRK.A) (NYSE: BRK.B) portfolio fall. Most of this decline came in June, with rising interest rates and inflation worrying investors that Mastercard's business could be negatively affected by a potential recession. Buffett stated in an interview with CNBC that he stopped buying only because Apple's share price rebounded.