The perpetual motion was originally conceived as a purely mechanical contrivance when once set in motion would continue to run forever. Such a machine would be merely a curiosity of no practical value and we know that the presence of friction makes it impossible. One would be of immense value is a machine producing a continuous supply of work without absorbing energy from the surroundings, such as a machine is called a perpetual motion machine of the first kind.

It is always possible to devise a machine to deliver a limited quantity of work without requiring a source of energy in the surroundings. For example, a gas compressed behind a piston will expand and do work at the expense of the internal work of the gas. Such a device cannot produce work continuously, however, for this to be achieved the machine must be capable of undergoing a succession of cyclic processes. An equation  ΣδQ=ΣδW states that if a net amount of heat is not supplied by the surroundings during a cycle, no net amount of work can be delivered by the system. Hence, the first law of thermodynamics implies that a perpetual motion machine of the first kind is impossible.