“Two things fill the mind with ever-increasing wonder and awe…the starry heavens above and the moral law within,” wrote Kant at the end of his Critique of Practical Reason (1788), and these word were inscribed on his tombstone. Kant was a highly influential German philosopher, who based his thinking on that of Descartes and Hume. One of his pupils wrote of him “Nothing worth knowing was indifferent to him,” and he undertook a huge project – he set out to examine the nature and limitations of human reason, and to find objective reasons for moral laws and duties. Although he thought it necessary to believe in God, Kant did not think you could prove his existence, and sought reasons independent of religious faith for the “moral law within” that he found so inspiring. He said he was not rejecting our ordinary moral judgements or producing a new principle, only a “new formula”, one that would enable us to work out the right course of action by thinking – something that humanists also try to do.
Kant’s formula, which he revised several times, is known as the “categorical imperative”, which states that we should act only in a way that we think anyone else in the same situation should also act: “Act only on that maxim which you can at the same time will to become a universal law.” So it would be wrong to lie, because we could not possibly will that everyone lied. Kant said that it was “categorical” – an absolute duty – to act on this principle of “ universalisability” He also said that it was our duty to treat other people as ends in themselves , never as means to an end, that is, we must not use them for our own purposes. While many humanists would agree with these principles, they would probably be less positive about Kant’s belief that consequences and emotions (even desirable one like love or compassion) were irrelevant to moral decisions, because they were not under our control.