The rationale behind the liberty tenet is that each member of society is the best judge of how to achieve their own happiness and fulfilment, and no-one (including the state) can claim to do it better. The tenet is intended to preserve an individual's freedom to pursue this personal notion of happiness, except where it restricts others' freedom to do the same. Some variation of this definition of liberty is integral to most political theories and ideologies, but it is best articulated by JS Mill, who argued that the only reason an individual or group may curtail the freedom of another is self-protection or the protection of others.

'The principle is, that 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 civilized community, against his will, is to prevent harm to others.

[Mill, On Liberty, 1859]

Regrettably, the principle is not as straightforward as it first appears. There are many scenarios where the terms Mill employs seem hazy. ‘Harm' most obviously relates to physical harm, but what about ‘potential' harm, such as inciting violence through actions or words? 'Self-protection' is also open to interpretation, especially if defined in aggregate terms — is it acceptable to strongly inhibit the freedom of an individual if it increases the potential safety of many? And where does one draw the line between what is harm and what is not? Difficult examples include nakedness in public, where the proponents may be causing only a negligible amount of harm, for instance in the form of embarrassment. Mill wanted to argue that ‘violations of good manners [...] may rightly be prohibited'. But his use of ‘may' is revealing; it is difficult to think of a systematic way to deal with grey areas. And there are further side-issues: what qualifies certain groups (children, the mentally handicapped) to not be included under this principle?

This tenet rightly underpins the laws and constitutions of most countries in the world, as well as the universal declaration of human rights. But the problems outlined above remain, manifesting themselves in a number of contemporary quandaries, including:

• The problem of free-speech and protest
• The problem of abortion and assisted suicide (Difficult cases of defining harm)
• The problem of self-inflicted harm (Obesity, smokers, and insurance. Who is responsible for their actions and who is not?)
• The balance between the state and the desire for safety (ID cards, privacy, the DNA database and Hobbes' social contract.)
• The potential for multiple legal systems, including religious ones (Sharia law and the court of chancery.)
• The problem of attire, in and out of public (Knives, turbans, nudity and the veil.)

The tenet of egalitarianism or equality has its roots in the assertion that everyone should have an equal chance of happiness. The assertion is a logical one: if everyone were to have different chances of happiness then it might be you — or people you care about — who are the ones who lose out. How far and by what means a state should enforce equality is a fundamental subject in political science and philosophy. Thinkers including Aristotle, Thomas Hobbes, Immanuel Kant and Karl Marx all spent time trying to find a satisfactory answer.

The most appealing method to date for working out what individuals should be equal in (and the one we use on this website) was proposed by the American philosopher John Rawls. Rawls' appeal to an Original Position (found in his 1971 work A Theory of Justice) is generally agreed to demonstrate that people should, at the least, be treated equally under the law and have equal opportunity to a happy and fulfilling life:

• Equality of law stipulates that there is one law that is equally applicable to everyone, regardless of gender, race or religion. This equality also manages to encapsulate the notion of equal liberty (because liberty is defined by the law).
• Equality of opportunity is the right for everyone to have an equal chance to be happy and fulfilled. Even one hundred years ago this would have been thought impossible, but this is now an explicit goal of most political parties around the world. See the post ‘Looking for ways to calculate equal opportunity' for more information about equality of opportunity and how it can be measured.

As with liberty above, egalitarianism is not without its share of irresolution, both within the tenet itself and with respect to the other tenets. Although most philosophers agree that the original position argument is convincing, many disagree about whether it shows that these are the only things individuals should be equal in.

• Meritocracy
• Freedom of religion
• Race definitions
• Jobs and discrimination under law
• Universal health care
• Education
• Business
• Marriage

Laws which make society more efficient often make the society better off. The efficiency tenet is intended to encapsulate the notion that improvements in efficiency increase everyone's chance of happiness. Efficiency is defined as the ratio of work produced to work put in.

The benefits of efficiency are well-known. In a situation where two people are marooned on a desert island, if one person specialises in catching fish and the other specialises in building shelter this would likely result in both having more time free to do other things compared with if each had to perform all tasks themselves.

Although the notion of job specialisation being more efficient than non-specialisation may seem an obvious one (you only need to set up equipment once, there's no time wasted switching between activities, and people's experience with the job grows a lot faster), it wasn't until the 19th century that it was proved mathematically. David Ricardo, working from the insights of the Scottish Enlightenment economist Adam Smith, demonstrated that, given certain assumptions, specialisation will always generate increases in efficiency compared to a society where there is no job specialisation. The study of ‘resource efficiency' in societies is called economics. Many economists spend their lives trying to prove that one way of organising or managing resources is more or less efficient than another. For instance, Smith argued that competition between companies in a market is a powerful and easy way to keep prices low, and thus the government should be wary of monopolies (where one company has no competitors). Economists since have tried to work out what policies and laws might make things more efficient, and what won't, and whether there is a system that would do better than market competition.

Policies and practices that increase efficiency have the potential to improve the quality of life of a huge number of people. Improvements in efficiency mean that most people in the world now have access to an ever increasing range of affordable goods and services. When not conflicting with the other tenets, policies that improve efficiency are to be selected over those that don't. As with the other tenets, there are various outstanding issues: methodological definitions remain unresolved (what should ‘work in' and ‘work out' be measured in?; how do we work out which policies lead to the most efficient system?; is there a limit to how efficient one can make society?) and it is not clear where to draw the line between improving efficiency and upholding the other tenets:

• Free markets
• Deregulation
• Population
• Debate

The tenet of custody stipulates that laws must balance the requirements of the current generation with the requirements of future generations. Although this applies to all laws (for instance, a bad law would be one that spent all the government money on things that will not be useful in the long term), it also extends to wider legislation like planning permission, population control, conservation, climate change, renewable energy and other sustainable policies. Ensuring laws meet the custody tenet is often problematic when trying to conform to the tenet of efficiency. Many of the things we want to keep to benefit future generations are not efficient, from. Without consideration for future generations, efficiency might require us to replace gas-lamps, Routemasters, country houses and forests with tarmac, roundabouts and plastic greenhouses. Would future generations be necessarily better off with these more efficient commodities? The answer is probably not, and careful future planning must underpin all good laws:

• Planning & Heritage
• The environment