The number of hours of sunshine that an area receives during the year depends on its geographic position on the Earth's surface (its latitude). For part of the day, the main source of the planet's energy, the sun, is hidden and night falls within a very well defined daily cycle. Mankind has tried to minimize this handicap since the dawn of history, and started by using fire to increase visibility at night. Today, lighting is in most cases the result of light produced by electricity.

In a society like Catalonia, the gesture of flicking a switch and turning on a light bulb, which takes the darkness away from a room, hardly seems like something special, when it is in fact a very recent phenomenon; indeed, even today there are some communities living in countries both near and far that have no electricity, which is a convenience that makes life seem "easier" and less "boring". What would we do without our fridge, washing machine, television and personal computer with an internet connection?

That is why we become so nervous when there is a power cut for some reason (such as the electric company not maintaining its lines properly, or a heavy snowfall). Of course, perhaps there are grounds for becoming nervous if your business depends on it, or if it is winter and the heating system is connected to the grid. Dependence is the word. Is this dependence excessive?

Something similar could be said of water. We turn on a tap and we can drink drinkable water (if the amount of lime or nitrates permit), we can fill a pot to cook food, we can take a shower and clean our teeth, we can do our laundry, we can water the garden, etc. Water reaches homes in Catalonia through supply networks, which avoids us having to travel varying distances from where we live to obtain it, which is something that happened in this country not so long ago, especially in rural areas. And while drinking water comes into our homes, the wastewater we produce leaves in the opposite direction. It is carried out through a network of sewers that runs towards a treatment plant in most cases. This service seems very normal, but it must be said that its widespread use in Catalonia is even more recent than the arrival of drinking water.

The population of Catalonia as a whole is part of the so-called welfare society. It is a very privileged part of the world's population that accounts for a very small percentage of the more than 7,000 million people on Earth. The combination of the dynamism of economic activities and financial and social demands made by workers have led to a rise in incomes, the establishment of reasonable working conditions with paid holidays, a range of basic public services such as education and health, and the creation of a system of aid for the most disadvantaged by governments. The economic crisis that began in 2007 is putting this system to the test. Democracy and human rights are also unmistakable signs of the welfare state, which may explain the late arrival of this "welfare" in Catalonia and the rest of Spain. However, much of the world's population does not have access to these basic services, lives under the poverty threshold, is engaged in regular armed conflicts, is dying of starvation, thirst or of diseases that would be cured in our welfare society, and is willing to leave everything behind to attempt to reach what it considers the land of opportunity at any price.

We live in a consumer society. Consumption is the driving force behind the motor of the economy, and when one gear in this engine has a problem the whole machine is affected. We consume commodities such as food, clothing and footwear, and we consume other items that have become basic, such as electric household appliances and cars, we consume leisure products, etc. In addition to consuming, we also communicate and travel. Passenger and freight networks facilitating accessibility have been created, while communication networks that enable contact with no need to travel have also been developed. However, consumption and travel also has an impact, especially of an environmental nature, and these are even more acute if the consumption is unnecessary and wasteful. Where does the balance lie between necessary consumption and measured consumption?

Activities that manufacture consumer goods or any product derived from the use of our world's natural resources also have an impact.

Bearing in mind that natural resources are finite, we must ask ourselves whether consumption (of water, oil, wood, etc.) is not excessive in societies like ours, and whether these levels of consumption might someday occur on a global scale. Economic development has enabled a high level of welfare to be achieved wherever it has taken place. Is our development sustainable?

The objective of the SDSUM project (Sustainable Development Science University Modules), of which this is the introductory module, is to attempt to answer the question above. However, before dealing with all these issues it is necessary to define - or at least attempt to define - what sustainable development means and how necessary it is to protect the environment and our health to ensure that our development is undoubtedly sustainable.