Probably everybody has some intuitive notion of what is meant by the term 'natural resources', but providing a rigorous and informative definition is not at all straightforward. At their simplest they might be defined as those economic factors in production or consumption which owe their origin and existence to natural phenomena, or to processes that occur autonomously in nature. This is an all-embracing statement which would seem to include anything that is neither man-made nor generated as a result of a manufacturing process that man has initiated - in short, everything that is neither 'capital' nor 'labour' in the classical economist's three-way classification of factors production. In this way all natural resources become equated with the traditional resource category of 'land', and this does capture one of their essential features - their initial availability precedes economic activity and is largely outside of man's influence. Natural resources are either God-given ('the original and indestructible properties of the soil') or originate from biological, chemical or geological processes that cannot at the present time be controlled at will.