Modern ethics has been chiefly concerned with the analysis of the conditions and principles of morality; and, in particular, one of its most important achievements has been the further elucidation of the Kantian dictum that “I ought” implies that I can. On the face of it Kant's contention seems perfectly straightforward, but, on examination, it becomes apparent that the simple word “can” covers a somewhat complicated ambiguity. When it is said that I ought to do act A, it may rightly be taken that I can do act A; but the question arises whether or not this in turn involves that I am aware that I can do act A. There is clearly some significance in the assertion that I can do many acts which, however, never occur to me, either because I am ignorant of certain facts or because I have a mistaken opinion about the facts. On the other hand, it may quite properly be said that I cannot perform any act of the possibility of which I am unaware. The problem then is to determine and examine the sense of the word “can” which is a pre-condition of moral obligation.