A free good is a good that is so readily available that it does not have to be economized. The concept is controversial. Murray Rothbard stated, "The common distinction between 'economic goods' and 'free goods' (such as air) is erroneous. As explained above, air is not a means, but a general condition of human welfare, and is not the object of action." According to Rothbard, general conditions are those elements of a person's environment "which he believes he cannot control and must leave unchanged". Ludwig von Mises agreed with this assessment, although with reference to the knowledge needed to convert resources into goods ready to consume, he states, "Such recipes are as a rule free goods as their ability to produce definite effects is unlimited. They can become economic goods only if they are monopolized and their use is restricted."