If you are in passionate love and want to celebrate your passion, read poetry. If your ardor has calmed and you want to understand your evolving relationship, read psychology. But if you have just ended a relationship and would like to believe you are better off without love, read philosophy.

Reasoning can take you wherever you want to go.

Like Plato, Kant believed that human beings have a dual nature: part animal and part rational. The animal part of us follows the laws of nature, just as does a falling rock or a lion killing its prey. There is no morality in nature; there is only causality. But the rational part of us, Kant said, can follow a different kind of law: It can respect rules of conduct, and so people (but not lions) can be judged morally for the degree to which they respect the right rules. What might those rules be? Here Kant devised the cleverest trick in all moral philosophy. He reasoned that for moral rules to be laws, they had to be universally applicable. If gravity worked differently for men and women, or for Italians and Egyptians, we could not speak of it as a law. But rather than searching for rules to which all people would in fact agree (a difficult task, likely to produce only a few bland generalities), Kant turned the problem around and said that people should think about whether the rules guiding their own actions could reasonably be proposed as universal laws. If you are planning to break a promise that has become inconvenient, can you really propose a universal rule that states people ought to break promises that have become inconvenient? Endorsing such a rule would render all promises meaningless. Nor could you consistently will that people cheat, lie, steal, or in any other way deprive other people of their rights or their property, for such evils would surely come back to visit you. This simple test, which Kant called the “categorical imperative,” was extraordinarily powerful. It offered to make ethics a branch of applied logic, thereby giving it the sort of certainty that secular ethics, without recourse to a sacred book, had always found elusive.