Freedom is an elusive concept because it means different things to different people. The fact that it can be interpreted differently is, ironically, down to the freedom we have over our own thoughts. For most people, the idea of freedom is best described as the ability to alter something – anything, in fact – freely. This is somewhat open-ended as a definition, however. As such, freedom tends to be referred to by contrasting it against its absence, in forms of social or economic control such as slavery, for example. When we are free of such constraints, we are said to be at liberty, that is the state in which we can act freely.