Pennsylvania Coal Co. v. Mahon
Supreme Court of the United States
260 U.S. 393 (1922)
While private property may be regulated to a certain extent, if regulation goes too far it will be recognized as a taking.
One fact for consideration in determining the limit of regulatory power is the extent of the diminution. When it reaches a certain magnitude, there must be an exercise of eminent domain and compensation paid to the owner.
Government could hardly go on if to some extent values incident to property could not be diminished without paying for every such change in the general law.
A strong public desire to improve the public condition is not enough to warrant achieving the desire by a shorter cut than the constitutional way of paying for the change.