The Buddha was characteristically resistant to dwelling on such speculative matters, and early opponents of Buddhism were quick to point out this apparent vulnerability in Buddhist thought.
Buddhists explain the difficulty using the analogy of fire: When one candle is used to light another, the new flame is not the same as the old flame, and yet the first flame directly causes the second. In the same way, one human life, with its particular accumulation of karma, gives rise to the next life, even though no permanent soul passes from one to the other