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[Article updated on 17 June 2019.]

For Aristotle, our unique capacity to reason is what defines us as human beings. Therefore, our , or our flourishing, consists in leading a life that enables us to use and develop our reason, and that is in accordance with reason.

Article 1 of the Universal Declaration of Human Rights (1948) states that all human beings are ‘endowed with reason’, and it has long been held that reason is something that God gave us, that we share with God, and that is the divine, immortal element within us. As per John 1:1: In the beginning was the Word (Greek, logos, ‘word’, ‘reason’), and the Word was with God, and the Word was God.

At the dawn of the so-called Age of Reason, Descartes doubted everything except his ability to reason. ‘Because reason’ he wrote ‘is the only thing that makes us men, and distinguishes us from the beasts, I would prefer to believe that it exists, in its entirety, in each of us…’

But what exactly is reason? Reason, for a start, is more than mere associative thinking, more than the mere ability to move from one idea (such as storm clouds) to another (such as imminent rain). Associative thinking can result from processes other than reason, such as instinct, learning, or . Reason, in contrast, involves providing reasons—ideally good reasons—for an association. And it involves using a system of representation, such as thought or language, to derive or arrive at that association.

People often amalgamate reason with logic, especially formal, deductive logic. At the very least, formal logic is seen as the purest form of reason. True, formal logic is basically an attempt to codify the most reliable or fail-safe forms of reasoning. But it is concerned merely with the validity of arguments, with the right relationship between premises and conclusion. It is not concerned with the actual truth or falsity of the premises or the merit and relevance of the conclusion.

Reason in contrast is a much broader psychological activity which also involves selecting and assessing evidence, creating and testing hypotheses, weighing competing arguments, evaluating means and ends, developing and applying (mental shortcuts), and so on. All this requires the use of judgement, which is why reason, unlike logic, cannot be delegated to a computer, and also why it so often fails to persuade. Logic is but a tool of reason, and it can sometimes be reasonable to accept something that is or appears to be illogical.

It is often thought, not least in educational establishments, that ‘logic’ is able to provide immediate certainty and the authority or credibility that goes with it. But logic is a lot more limited than many people imagine. Logic essentially consists in a set of operations for deriving a statement from other statements. In a sense, it merely makes explicit that which was previously implicit. It brings nothing new to the table. The conclusion merely flows from the premises as their inevitable consequence:

All birds have feathers. (Premise 1) Woodpeckers are birds. (Premise 2) Therefore, woodpeckers have feathers. (Conclusion)

Another major problem with logic is that it relies on premises that are founded, not on logic itself, but on inductive reasoning. How do we know that ‘all birds have feathers’? Well, we don’t know for sure. We merely suppose that they do because, so far, every bird that we have seen or heard about has had feathers. But the existence of birds without feathers, if only in the fossil record, is not beyond the bounds of possibility. Many avian species are hatched naked, and a featherless bird called Rhea recently took the Internet by storm. A lot also depends on how and how tightly we define our terms, especially, in this case, ‘bird’. There are in fact several definitions of what a bird might be, including, for example, ‘all archosaurs closer to birds than to crocodiles’ and ‘advanced archosaurs with feathers’…

Inductive reasoning only ever yields probabilistic ‘truths’, and yet it is the basis of everything that we know or think that we know about the world we live in. Our only justification for induction is that it has worked in the past, which is, of course, an inductive proof, tantamount to saying that induction works because induction works! To rescue it from this Problem of Induction, Karl Popper argued that science proceeds not inductively but deductively, by making bold claims and then seeking to falsify those claims. But if Popper is right, science could never tell us what is, but only ever what is not.

Putting these inductive/deductive worries aside, reason is limited in reach, if not in theory then at least in practice. The movement of a simple pendulum is regular and easy to predict, but the movement of a double pendulum (a pendulum with another pendulum attached to its end) is extremely chaotic. If you’re interested, there are videos of double pendula on the Internet. Similarly, the interaction between two physical bodies such as the sun and the earth can be reduced to a simple formula, but the interaction between three physical bodies is much more complex—which is why the length of the lunar month is not a constant and the length of any one lunar month is extremely difficult to approximate. But even Three-Body Problems are as nothing compared to the entanglement of human affairs. God, it is sometimes said, gave all the easy problems to the physicists.

The intricacies of human affairs often lead to a paralysis of reason, and we are left confused and undecided, sometimes for years or even into the grave. To cut through all this complexity, we rely heavily on forces such as emotions and desires—which is why Aristotle's Rhetoric on the art of arguing includes a detailed dissection of what used to be called the passions. Our emotions and desires define the aims or of our reasoning. They determine the parameters of any particular deliberation and carry to conscious only a small selection of all the available facts and counterfactuals. Brain injured people with a diminished capacity for emotion find it especially hard to make decisions, as do people with apathy, which is a symptom of severe and other mental disorders. Relying so heavily on the emotions comes at a cost, which is, of course, that emotions aren't rational and, moreover, can distort reasoning. alone can open the gate to all manner of self-deception. On the other hand, that emotions aren't rational need not make them irrational. As I argue in my new book, Hypersanity: Thinking Beyond Thinking, some emotions are appropriate or justified, while others are not. This is why, as well as coming to grips with maths and science, it is so important to educate our emotions.

Another shortcoming of reason is that it sometimes leads to unreasonable conclusions, or even contradicts itself. In On Generation and Corruption, Aristotle says that, while the opinions of certain thinkers appear to follow logically in dialectical discussion, 'to believe them seems next door to madness when one considers the facts'. In Plato's Lesser Hippias, Socrates manages to argue that people who commit injustice voluntarily are better than those who do it involuntarily, but then confesses that he sometimes thinks the opposite, and sometimes goes back and forth:

My present state of mind is due to our previous argument, which inclines me to believe that in general those who do wrong involuntarily are worse than those who do wrong voluntarily, and therefore I hope that you will be good to me, and not refuse to heal me; for you will do me a much greater benefit if you cure my soul of ignorance, than you would if you were to cure my body of disease.

The sophists of Classical Greece taught rhetoric to wealthy young men with ambitions of holding public office. Prominent sophists included Protagoras, Gorgias, Prodicus, Hippias, Thrasymachus, Callicles, and Euthydemus, all of whom feature as characters in Plato’s dialogues. Protagoras charged extortionate fees for his services. He once took on a pupil, Euathlus, on the understanding that he would be paid once Euathlus had won his first court case. However, Euathlus never won a case, and eventually Protagoras sued him for non-payment. Protagoras argued that if he won the case he would be paid, and if Euathlus won the case, he still would be paid, because Euathlus would have won a case. Euathlus, having picked up a trick or two from his master, retorted that if he won the case he would not have to pay, and if Protagoras won the case, he still would not have to pay, because he still would not have won a case!

Whereas philosophers such as Plato and Socrates use reason to arrive at the truth, sophists such as Protagoras abuse reason to move mobs and enrich themselves. But we are after all social animals, and reason evolved more as a means of solving practical problems and influencing people than as a ladder to abstract truths. What’s more, reason is not a solitary but a collective enterprise: premises are at least partially (and often entirely) reliant on the achievements of others, and we ourselves make much better progress when prompted and challenged by our peers.

The principal theme of Plato’s Protagoras is the teachability of virtue. At the end of the dialogue, Socrates remarks that he began by arguing that virtue cannot be taught, only to end by arguing that virtue is no other than knowledge, and therefore that it can be taught. Protagoras, in contrast, began by arguing that virtue can be taught, but ended by arguing that some forms of virtue are not knowledge, and therefore cannot be taught! Had they not debated, both men would have stuck with their original, crude opinions and been no better off.

Why does reason say ridiculous things and contradict itself? Perhaps the biggest problem is with that old chestnut, language. Words and sentences can be vague or ambiguous. If you remove a single grain from a heap of sand, it is still a heap of sand. But what happens if you keep on repeating the process? Is a single extant grain still a heap? If not, at what point did the heap pass from being a heap to a non-heap? When the wine critic Jancis Robinson asked on Twitter what qualifies someone to call themselves a sommelier, she received at least a dozen different responses. Similarly, we might say to someone something like, ‘You can’t do that. Well, you can, but…’

Another big problem is with the way we are. Our senses are crude and limited. More subtly, our minds come with built-in notions that may have served our species well but do not accurately or even approximately reflect reality. Zeno’s paradoxes, for example, flush out the limits of our understanding of something as rudimentary as movement. Some of Zeno’s paradoxes side with quantum theory in suggesting that space and time are discrete, while others side with the theory of relativity in suggesting that they are continuous. As far as I know (I’m not a physicist), quantum theory and the theory of relativity remain completely unreconciled. Other concepts, such as infinity or what lies outside the universe, are simply beyond our ability to conceive.

A final sticking point is with self-referential statements, such as ‘This statement is false’. If the statement is false, it is true; but if it is true, it is not false. But let’s not open that can of worms.

Despite its shortcomings, I hold reason in the highest regard. It is after all the foundation of our peace and freedom, which are under constant threat from the blind forces of unreason. In highlighting the limits of reason as I have just done, I seek not to disparage or undermine it but to understand and use it more effectively, and even to revel in it.

‘The last function of reason’, said Blaise Pascal, ‘is to recognize that there are an infinity of things which are beyond it. It is but feeble if it does not see so far as to know this.’

Related articles by the same author: The Problem of Knowledge, The Problems of Science, What is Truth?