||Some statements are not only true but necessarily true. The fact they state can not possibly be otherwise. Consider the necessary "My sister is a woman" versus the non-necessary (i.e., accidental or contingent) "I have a brother". There are, however, different kinds of necessity: a logical necessity, like "It is raining or it is not raining", has its source in the rules of logic; a conceptual necessity, like "Bachelors are unmarried men", is true in virtue of the meanings of the words of the statement; a metaphysical necessity, like "Water is H2O", is said to be necessarily true because it is the essence of water to be H2O. Nomological necessity, finally, is grounded in or grounds the laws of nature: "Increased heat at constant volume necessitates (or causes) higher pressure". Some philosophers first give a theory of what laws of nature are and then can say on that basis that events happen with nomological necessity when they happen because of the laws. Other philosophers take nomological necessity to be basic and define law statements in terms of nomologically necessary relations in nature.