Reason is more than a tool

If intelligence is merely optimisation then machines will outrun us. Kant tells us why human reason is so much more

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is associate professor of philosophy at the Pontifical Catholic University of Chile in Santiago and a visiting scholar at the University of California, Berkeley. She is also the philosopher-at-large for Prospect magazine, where she has a monthly column.

A few months ago, I sat in a packed university auditorium listening to a celebrated computer scientist explain the brilliance of large language models (LLMs). These systems optimise, he said. And they improve at optimisation on the basis of past experience. He paused. That, he said with a smile, is intelligence. The audience nodded along approvingly. Meanwhile, a philosophical alarm bell began sounding in my head.

Not because the speaker’s claim was implausible. On the contrary, as a description of much contemporary artificial intelligence, it seemed apt. AI systems are built around objectives, losses, rewards, predictions, preferences, benchmarks and feedback loops. They are trained to perform better relative to some standard of success. In one form or another, they optimise. That is a great part of their technical power.

What troubled me was the ease with which a definition appropriate to machines sounded like a definition of intelligence as such. That slide from engineering model to human anthropology is easy to miss. We begin by saying: this is how machines work. Then, almost without noticing, we say: this is what intelligence is. And then, more dangerously still: perhaps this is what we are.

An old argument about human beings had entered the room without announcing itself. It sounded, instead, like a definition. The AI revolution has brought one of the Enlightenment’s deepest quarrels back to life – and made one side of it seem like plain common sense. In my view, the wrong side. Hence the alarm bell.

In the 18th century, philosophers were concerned not only with the question of what reason is, but with what it is good for. Is reason essentially a servant, a clever instrument for achieving ends supplied from elsewhere? Or is it sovereign, capable of judging, revising and even legislating ends for itself? Can it ultimately shape the direction of human life, or is it merely along for the ride? Everything – from science to social progress to the viability of the French Revolution – seemed to hang in the balance.

David Hume, the cool-eyed Scottish anatomist of human nature, gave the most famous answer on one side: ‘Reason is, and ought only to be the slave of the passions,’ he wrote in A Treatise of Human Nature (1739). It can ‘never pretend to any other office than to serve and obey them.’ Reason does not command. It defers. It does not tell us what to want or what might be worth wanting. Instead, it tells us how to get what we already want. It calculates relations, weighs evidence, discovers causes and effects, draws inferences, corrects mistakes about means. But it cannot judge or generate ends. If I desire revenge, reason can help me sharpen the knife. If I desire peace, it may recommend compromise. Passion sets the destination. Reason merely shows the way there.

Hume was quite serious about this. It is ‘not contrary to reason to prefer the destruction of the whole world to the scratching of my finger,’ he wrote. That sentence still shocks, as it was meant to. His point is that reason’s practical use is limited to exposing false beliefs and imprudent means. When it comes to action: reason is servant, not master.

This view is often treated as too dark and cynical. That is unfair to Hume. For him, passions were not merely appetites or primitive urges. They included sympathy, benevolence, moral feeling, concern for others, love of tranquility, even a kind of longing for freedom from our more violent desires. To Hume’s further credit, his view has strong intuitive appeal. He was right that reason often serves passion, and right to give feeling, sociability and temperament their due. His mistake was to conclude that reason can do nothing more. By making passion so rich and reason so poor, Hume left us with a lopsided picture of human nature: ennobling about what the passions can be and accomplish, but implausibly restrictive about reason. Without reason’s power to project, assess and revise ends, we may know how to reach a destination, but not whether it is one worth choosing.

To act freely is to let a possible future, grasped under an idea of what ought to be, transform the present

Immanuel Kant, the Königsberg revolutionary who did more than anyone else to set the terms of modern ethics, understood this very well. Writing a few decades after Hume’s Treatise, Kant forcefully rejected the servant model of reason. He accepted Hume’s warning that reason cannot simply read values off the furniture of the world. But he denied that our ends must therefore simply be determined by desire, temperament or social convention. For Kant, reason is not only a faculty for choosing efficient means to independently given ends. It is also the power to ask whether an end is worthy of us in the first place, whether it can be justified, whether it can be willed without contradiction, whether pursuing it treats others as persons or as mere tools.

This is where Kant’s richer notion of reason as freedom enters. A merely mechanical being is pushed forward and determined by prior states, be they desires, biological impulses, evolutionary drives or something similar. A rational being, by contrast, can act for the sake of an imagined but unrealised future it itself originates and proposes. I have the power to represent a possible world – a friendship repaired, an injustice remedied, a child educated, a law reformed – and act to bring it about, even when there is no empirical evidence that such a world has existed before. I have the power to invent something new. Practical reason, Kant writes, does not deal with objects in order to know them, but ‘in order to make them........

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