When laying down the framework for the basic debate between positivism and natural law, Hart thinks it necessary to clarify, first, what the debate is not about. He does not contest the truistic claim that the development of law, at all times and places, has been profoundly influenced by conventional (and, for that matter, unconventional) morals. However, that does not necessarily imply that a legal system must exhibit some specific conformity with morality or justice.
Hart's argument is essentially a response to classical natural law theorists. Such theorists believe that there is a specific system of morality, consisting of certain principles, discoverable by reason, that any law must conform with if it is to have the status of law. This viewpoint is grounded in the basic idea that Hart defines as the "teleological view of nature"; namely, that everything in nature, including man, is moving towards a teleos, a specific end. The teleos of an acorn would be the culmination of its growth into a full-fledged, healthy oak tree. The teleos of human beings is a condition of bilogical maturity coupled with an excellence of mind and character. And natural law must promote, by its directives, the achievement of this teleos.
The idea of what forms teleos for mankind has been violently disputed and debated over the centuries. Taking on from Hobbes and Hume, Hart chooses what he feels is the lowest common denominator, the basic indisputable minimum which (almost!) all people will agree on: survival, nothing more, nothing less.
There are two reasons that Hart provides for picking survival as the basic end for humankind. First, that not only do almost all men wish, above all else, to continue living, but the very structures of our thought and language (words like harm and benefit, illness and cure, disease and safety) embody this desire. Second, while specifically discussing law, we are discussing how best to govern the conduct of people who are living together; and to achieve that, they must be living in the first place!
Therefore, given that survival is an undisputed end, laws must contain certain content to make sure that that end is realised. This Hart calls the minimum content of natural law.
Therefore, Hart's idea of the minimum content of natural law can be defined in the following manner:
"Proposition 1: Given that law is a mechanism for regulating the behaviour of individuals in a social association, it must have a certain basic, minimal content in order that the association be viable.
Proposition 2: In turn, for the ssociation to be viable, the survival and continued existence of at least some of its members must be ensured.*
Proposition 3: The content which ensures such survival is called the minimum content of natural law."
Obviously, the content of such law is based on certain facts grounded in human nature and the state of human existence. Hart lists five such facts:
Human vulnerability, which entails a restriction on the free use of violence;
Approximate equality, which again, restricts the use of aggression;
Limited altruism, which requires systems of mutual forbearancec;
Limited resources, which require some system of property;
Limited understanding and strength of will, which require some form of sanctions.
These five truisms about human nature, claims Hart, makes it a "natural necessity" that law has a certain content that embodies the minimum forms of protection for persons, property and promises.
Lastly, it may be noted that Hart's idea of there being a minimum content of natural law strongly resembles Fuller's idea of a "morality of duty," and the "eight desiderata" of law that make a legal system possible.
Does Hart make an impermissible leap from the "is" of human nature to the "ought" of the minimum content of natural law while grounding this content in the human condition? One may argue that the minimum content of natural law is dependant upon survival being an end; and that, in turn, is dependant upon the five facts that Hart has pointed out; and therefore, in the latter part of the argument, the impermissible leap from fact to normative claim has taken place.
It is important to note, however, that a certain proposition may be contingent upon another, but there need not be a causal link between the two. Consider the following:
Proposition A: I will have tea with X at his home tomorrow evening at 5 PM.
Proposition B: X will be at home tomorrow evening at 5 PM.
The truth value of the first statement will change in accordance with a change in the truth value of the second statement. However, my having tea with X is not "caused" by X being at home. Therefore (as Finnis points out), while it is true that the moral content of law is contingent upon the nature of human existence, and would change as the latter changes, that does not equate to deriving morals out of facts.
* Because then no man would voluntarily cooperate within the system, and for a legal system to survive, there must be the cooperation of at least some of its members (for whatever reasons) in order to oppress the coerced majority.