Natural law theory is not a single theory of law, but the application of ethical or political theories to the questions of how legal orders can acquire, or have legitimacy, and is often presented as a history of such ethical and political ideas.
These theories would explained the nature of morality, thus making natural law theory a general moral theory. The basic idea was that man could come to understand, either by his own reasoning or help from God, how he should act rightly in respect of his fellow man. However, within modern jurisprudence, much of the importance of natural law has been eroded from a question on the meaning of justice or how a system of law could be understood as legitimate; into a question of what is the relationship between natural law theories and the everyday operations of a legal system.
This is because much of natural law has been savaged by two criticisms: a. Natural law theories' assertion that in order to understand what law is, it is necessary to involve oneself in an exploration and explaination of what law ought to be is inherrently faulty. (the fact/value distinction)
This confuses the description (law's actual existence) with prescription (the evaluation of law as good or bad). Natural law theories stray between the logically unconnect fields of meaning of 'is' and 'ought', which is dubbed the 'naturalistic fallacy'.
The incompatibility between these two fields is illustrated by the classic interpretation of Hume's law, that one cannot derive a statement about what ought to be from a statement about what is, or vice versa. To give an example of the non sequitur involved in this kind of reasoning is that the fact that only women can bear children, points to the conclusion that they This supposed link between the capacity to bear children and motherhood is provided through social conventions and is entirely contingent. The former does not follow the latter as if it was some sort of natural and unavoidable consequence, and thus cannot be seen to somehow be an inherent property of human beings and the way they organise their world.
b. Any attempt to identify a necessary common element of ethics in all legal ststems appear to founder on the difficulty of agreeing on a common set of ethical values, as coherent moral values are extremely difficult to articulate, let alone to prove.
c. The source of natural law
It can be discerned from Cicero's works that there are two very different sources of natural law: i. Our shared reason
Classical and Modern Natural Law Theory
"and it is not only justice and injustice that are distinguished naturally, but in general all honourable and disgraceful acts. For nature has given us shared conceptions and has so established them in our minds that honourable things are classed with virtue, disgraceful ones with vice"
These are our shared conceptions’ given us by nature by which we all classify things in the same way, evil with evil, good with good.
ii. God as the author of natural law
"and there will not be different laws at Rome and at Athens, or different laws now and in the future, but one eternal and unchangeable law will be valid for all nations and all times, and there will be one master and ruler, that is, God, over all of us, for he is the author of the law, its promulgator, and its enforcing judge."
Is it not possible for our reason to conflict with what we learn from the revelation of God’s will? Was the moral law as revealed by God good just because God willed it, or was it willed by God because it was good? There is a circularity in attemption to define morality in terms of God's will. To say that God deserves our obedience because he is morally perfect can only make sense if we understand the notion of moral perfection before we relate it to God. This had led natural lawyers such as Grotius to say that natural law was willed by God, but, but was willed by him because it is...