  We propose a causal framework to explain the catastrophic forgetting in
Class-Incremental Learning (CIL) and then derive a novel distillation method
that is orthogonal to the existing anti-forgetting techniques, such as data
replay and feature/label distillation. We first 1) place CIL into the
framework, 2) answer why the forgetting happens: the causal effect of the old
data is lost in new training, and then 3) explain how the existing techniques
mitigate it: they bring the causal effect back. Based on the framework, we find
that although the feature/label distillation is storage-efficient, its causal
effect is not coherent with the end-to-end feature learning merit, which is
however preserved by data replay. To this end, we propose to distill the
Colliding Effect between the old and the new data, which is fundamentally
equivalent to the causal effect of data replay, but without any cost of replay
storage. Thanks to the causal effect analysis, we can further capture the
Incremental Momentum Effect of the data stream, removing which can help to
retain the old effect overwhelmed by the new data effect, and thus alleviate
the forgetting of the old class in testing. Extensive experiments on three CIL
benchmarks: CIFAR-100, ImageNet-Sub&Full, show that the proposed causal effect
distillation can improve various state-of-the-art CIL methods by a large margin
(0.72%--9.06%).
