As the word itself suggest, we are here concerned with particular cases. When the moral law or the principles of ethics are applied, they are applied to particular cases, each one unique in the circumstances and factors that are operative in the here and now.
Aristotle in Book V of his Ethics, on justice, points out that general rules do not apply perfectly to particular cases; and so an equitable dispensation from the general rule is required to do justice in the particular case.
In the tradition of the Anglo-American common law, the separation of courts of law from courts of equity, which were the province of the Chancellor, provided an institution that enabled justice to be done in the particular case, justice that departed from the general rule.
One way of saying what is sheer dogmatism in the ethics of Immanuel Kant is to point out that his moral law -- his so-called categorical imperative -- completely ignores the circumstance of particular cases. According to Kant, there are no exceptions whatsoever to the general rule that lying violates one's moral duty to tell the truth.
We are to imagine the following case. A man is standing at his fence on the roadside. He sees an individual breathless and haggard with fear running down the road, as if pursued. A little beyond his house, the road branches into two forks, one to the left and one to the right. The individual running away pauses for a moment and then decides to take the fork to the left.
A moment later, two villainous-looking individuals brandishing big clubs appear and ask the man who is still standing at his fence whether the man they are pursuing with deadly intent came by and, if so, which fork in the road beyond the house he took.
Should the onlooker tell them the truth though he can be almost certain that if the pursuers catch the man who is fleeing, they will do him in with their clubs and fists?
Without knowing whether the individual who is fleeing from his pursuers is guilty or innocent of some crime, and without knowing anything about the motivation of the pursuers. Kant answers the question of whether the onlooker should tell the truth flatly in the affirmative. Kant does not allow for any casuistry whatsoever. No moral philosophy that does not provide casuistry for finding exceptions to general rules can be sound.
There are many other reasons for finding fault with the moral philosophy of Immanuel Kant, but the dismissal of casuistry is sufficient in itself to challenge the validity of Kantian ethics.