Few of Kant's doctrines are as difficult to understand as that of self-affection. Its brief career in the published literature consists principally in its unheralded introduction in the Transcendental Aesthetic and unexpected reappearance at a key moment in the Deduction chapter in the second (B) edition of the first Critique. After blazing its trail, self-affection retreats into the background, with a discussion befitting its importance occurring only in the unfinished Opus postumum. This step out of the limelight, however, belies the doctrine's continued importance for Kant; indeed, Kant seemed to think that in self-affection was to be found the key to the project that occupied him in his last years. Thus, ‘the possibility of the transition from the metaphysical foundations of natural science to physics does not consist in the fact that the subject is empirically affected but rather that it affects itself’ (Opus postumum, 22: 405). As he continued to struggle with this doctrine and with the pivot-point on which to work this vital transition, Kant himself would surely come to rue his confident statement in the B Deduction: ‘I do not see how one can find so many difficulties in the fact that inner sense is affected by ourselves’ (B156n).