Why did Immanuel Kant believe lying is never morally acceptable?

Immanuel Kant maintained that lying is never morally acceptable because he believed morality is rooted in our capacity for free, rational choice. From his perspective, lying is an assault on morality itself, as it aims to undermine this rational capacity in others.1

Kant’s ethical framework, known as deontological ethics, rejects the idea that the morality of an action should be judged by its consequences. Instead, he argued that actions must conform to a categorical imperative—a moral law that applies to all rational beings regardless of personal desires.21

He proposed two primary reasons for why lying is inherently wrong:
* Universal Law: A maxim such as "make false promises when it suits you" cannot be willed as a universal law because it is self-defeating; if everyone lied, the practice of promising would lose all meaning and credibility.23
* Humanity as an End: Kant argued that we must treat humanity always as an end and never merely as a means. Lying treats others as a means to achieve one's own goals, thereby violating their autonomy and dignity.21