Why set standards that no one can achieve?
A rule that bends every time it is tested is not a rule; it is a memory edited after the fact.
If a standard can never be fully achieved, it can look pointless like pouring water into a sieve. But the argument in Groundwork for the Metaphysics of Morals by Immanuel Kant is not that humans will meet the standard perfectly. The argument is that without such a standard, the idea of justification itself collapses.
When you evaluate an action, you need some criterion that is not just your mood, fear, or advantage in that moment. If the rule changes every time circumstances pressure you, then there is no difference between reasoning and rationalizing. Kant is trying to prevent that slide. The standard acts like a measuring line that stays fixed while your situation changes.
It is similar to how navigation works. A compass does not guarantee that you will walk in a perfectly straight line across mountains, storms, or obstacles. But without the compass, you are no longer navigating you are just moving and later inventing a story about why the direction made sense. So the standard is not meant to be a target you hit perfectly. It is meant to prevent self-justification from rewriting the rule after the action. That is the key argument.
There is also a second reason that is more practical than it first appears. Many systems only work if participants orient themselves toward a strict rule even if they sometimes fail to meet it. Think of aviation safety, medicine, engineering, or education. If the rule were lowered to “do what is realistically achievable under pressure,” reliability would drop fast because people would adjust the rule downward each time difficulty appears. High standards act as stabilizers, not because humans meet them perfectly, but because they slow down the erosion of responsibility. The rule for teachers about protecting children first is not there because every teacher will perform perfectly in every emergency. It exists so that when a crisis happens, the orientation is already decided. Without that prior rule, instinct and chaos take over. Kant wants to keep the rule from dissolving into convenience.
To put it bluntly, humans are very good at adjusting principles to fit what they already want to do. A strict standard is a defense against that habit. So the real question is if the standard disappears, what replaces it? Usually the answer is not realism but improvisation mixed with self-interest. That is why Kant insists on standards that appear severe. They are meant to resist the constant pressure of human adjustment.