Ten of the greatest: Philosophical principles

1. THE HARM PRINCIPLE

by JOHN STUART MILL, 1806-1873

Whenever legislation is proposed that limits our freedoms, someone will reach for Mill's On Liberty and point to the passage that says, 'The sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is self-protection. That the only purpose for which power can be rightfully exercised over any member of a civilised community, against his will, is to prevent harm to others. His own good, either physical or moral, is not sufficient warrant.' What could be clearer? Except it isn't clear: it depends on what you mean by harm. Does hate speech harm minorities? Does sexist language harm women, by making them less credible in the eyes of society? Philosophical principles are like credit agreements: the headlines are convincing, but the small print catches you out.

Whenever legislation is proposed that limits our freedoms, someone will reach for John Stuart Mill's On Liberty

2. THE PRINCIPLE OF SUFFICIENT REASON

GOTTFRIED LEIBNIZ 1646- 1716

The idea that everything is as it is for a reason is the assumption behind most of philosophy. If we thought that things just happened, we would not bother to try to work out their causes. But then nor would we assume that longer days meant more sunshine meant warmer weather. But this principle is crucially different from the one that says everything must have a purpose. There must be a reason why the big bang happened, but that does not mean it happened for any end or goal.

There must be a reason why the big bang happened, but that does not mean it happened for any end or goal

3. THE MEAN

by ARISTOTLE, 382BC- 322BC

Moral thinking is steeped in sharp dualities: Good v Evil, God v Satan, Right v Wrong, Heaven v Hell. Popular mythology, from humanity's fall from grace in the Garden of Eden to Star Wars, is full of tales of people going over to the dark side. But long before modern psychology told us that we all have our shadow side, an Ancient Greek philosopher came up with an idea that was even more subtle: it is not that there are shades of grey between moral black and white - good and bad aren't opposites at all. Rather, the good is a 'mean' that stands between two bads: that of excess and that of deficiency. Courage, for instance, is the mean between the excess of rashness and the deficit of cowardice. Mercy is the mean between the excess of vengefulness and the deficiency of surrender. It's a brilliant idea that utterly transforms how you look at right and wrong.

Aristotle's saw good as a 'mean' that stands between two bads: that of excess and that of deficiency

4. THE FALSIFICATION PRINCIPLE

by KARL POPPER, 1902-1994

Common sense once held that a theory was scientific if you knew how to prove it. But Popper suggested that a theory is only scientific if you know what would disprove it. That's why conspiracy theories are nonsense: no matter what the evidence, believers insist this proves how tough the cover-up is. Similarly, you could argue that the theory that God does what is best for us is not scientific, because whatever happens, believers insist it must be for the greater good. God's goodness may be a theological claim but it's not evidential.

Karl Popper suggested that a theory is only scientific if you know what would disprove it (Above, the film Capricorn One)

5. OUGHT IMPLIES CAN

by IMMANUEL KANT, 1724-1804

How often do people insist that 'Something should be done' even though they've no idea what that something is? But unless you have an idea what should be done, how do you even know that it's possible to do anything at all? It makes no sense to say something should be unless it actually can be. Kant is usually credited with formulating this principle: 'Since reason commands that such actions should take place,' he wrote, 'it must be possible for them to take place.' In other words, if a prescription is truly rational, then it must be possible. Which means if it looks rational, but isn't possible, it isn't rational at all, like expecting a system to run on debt indefinitely.

Immanuel Kant said: 'Since reason commands that such actions should take place, it must be possible for them to take place'

6. THE PRINCIPLE OF EVIDENCE

by DAVID HUME, 1771-1776

'A wise man proportions his belief to the evidence' sounds like advice you know already. But it's more easily agreed with than followed, and the results can be uncomfortable. No wonder David Hume felt the need to restate it. In his essay Of Miracles he says: 'A weaker evidence can never destroy a stronger'. Sounds obvious. But when it comes to the miraculous, has the testimony of any witness ever been stronger evidence than the testimony of all the rest of life, which tells us that nature's laws do not admit exceptions? If not, says Hume, then anyone who claims to base belief on evidence can never believe in miracles.

In David Hume's essay Of Miracles, he says: 'A weaker evidence can never destroy a stronger'

7. THE PRINCIPLE OF CHARITY

by DONALD DAVIDSON 1917 -2003

Some principles cannot claim to have a single author. The principle of charity is one, although the 20th-century U.S. philosopher Donald Davidson has more claim than most to have set it out clearly and systemically. In its broadest sense it urges us to interpret the meaning of what others claim in ways that make them as rational as possible. It is, if you like, an injunction to give others the benefit of the doubt. If what someone says could be plain wrong or just badly phrased, assume the latter, until proven otherwise. It is, of course, the opposite principle to that followed by politicians, newspaper columnists and militant atheists, all of whom assume their opponents to be bone-headed fools. In other words, it is a principle more needed now than ever.

The principle of charity urges us to interpret the meaning of what others claim in ways that make them as rational as possible (Above, Tom Hanks in  )

8. THE DIFFERENCE PRINCIPLE

by JOHN RAWLS, 1921 -2002

If some philosophical principles seem so obvious as not to be worth stating, others shatter consensus. Such is the case with Rawls's difference principle, which the Liberal Democrats once considered adopting as policy. This states that increases in inequality are only permissible if they benefit the worst-off members of society. It sounds radical, but after recent events with bankers' bonuses, fewer people now believe that concern about growing pay at the top of the economic scale is pure envy. The difference principle states that it is fine for the rich to get richer only if the poor always become richer than they would have done had the wealthy been held back. It's a liberal compromise between the socialist demand for equality and the neo-liberal disregard for equality.

The difference principle states that increases in inequality are only permissible if they benefit the worst-off members of society

9. JUST WAR

by THOMAS AQUINAS, 1225 -1274

Thinkers have long pondered when war is morally justified, but modern theories of just war are little more than amendments to those set down by Thomas Aquinas in the 13th century. The morality of a war is determined both by the reasons for starting it and its conduct once begun. To wage war, the cause needs to be just, it must be waged by an army with legitimate authority, it must be fought for the right intentions, there must be a good probability of success, the response must be proportionate, and it must be the last resort. Armies must use proportionate force and discriminate between combatants and innocent civilians.

The morality of war is determined both by the reasons for starting it and its conduct once begun

10. OCCAM'S RAZOR

by WILLIAM OF OCCAM, 1288-1348

In an age of five-blade razors, the one wielded by William of Occam is ironically named, for it is the requirement not to multiply entities beyond necessity. So, if you can explain the workings of the world by postulating only the existence of matter, you should not prefer an explanation that also posits ectoplasm, unless that more complicated theory can explain more. The principle is also known as one of economy of explanation: all other things being equal, a simpler explanation is more likely to be true than a complicated one. It's a principle that has even found its way into Sex And The City: if a man is sending a woman mixed messages, the simple answer is he's just not that into her.

The Occam's razor principle has even found its way into Sex And The City: if a man is sending a woman mixed messages, the simple answer is, he's just not that into her