Kant draws a fundamental divide between things and persons. Things have conditional worth — they are valuable insofar as they serve some purpose, and can always in principle be replaced by something of equivalent utility. Persons are different. Their existence is not conditional on their usefulness. They are rational beings, and rational beings are ends in themselves — they carry their worth within them.
The formula does not forbid using others in the ordinary sense — we use the baker to get bread, the doctor to get treatment. What is forbidden is treating someone merely as a means: using them in a way that bypasses or overrides their rational agency, as when we deceive them or coerce them. The key word is "merely." We may use people while also respecting them as rational agents with their own purposes.
This formulation grounds Kant's account of human dignity. Because rational nature is an end in itself, it cannot be assigned a price — it is beyond all valuation. This is why no calculation of consequences, however favourable, can justify treating a person merely as an instrument. The formula of humanity gives moral force to the intuition that some things simply cannot be done to people, whatever the outcome.
The formula of humanity appears in the Second Section of the Groundwork, where Kant tests it against the same four examples he uses for the formula of universal law.
