Why would all law be positive law?

Question

Oliver Wendell Holmes, Jr., in his 1897 The Path of Law, argues that law does not relate to objective reality but is created by whatever judges decide. Thus, according to him, all law is positive law, a manmade convention. This is diametrically opposed to natural law theory, which says that there are laws built into nature, e.g., laws of physics, laws of human nature, etc. (cf. St. Thomas Aquinas's Treatise on Law).

The OED distinguishes positive and natural law when defining "positive law"

Formally laid down, imposed, or decreed; proceeding from enactment or custom; conventional. Originally and chiefly in positive law n. (in political and legal philosophy, ethics, and theology) a law or body of laws artificially instituted or imposed by an authority, often as contrasted with natural law rooted in the requirements of justice and right reason (cf. natural law n.).

Thus, why do Oliver Wendell Holmes, Jr., and those who follow his legal philosophy think that all law is positive law?

Answer 1

There's a few different questions (at least from the perspective of those who give natural law some role) that are tangled up in your question. And if few parse some of these, Holmes' views should be clearer (for full disclosure, I haven't read Holmes but have studied the natural/positive law distinction within philosophy).

Question #1 How does something become a law for a given nation?

Positive law answer: judges or legislators make it a law for that nation.

Natural law answer: people use reason and look at nature and then make that into a law for their nation.

In a sense, the positive law answer is cleaner because it bypasses why, and the natural law answer to some extent agrees completely with that why-less answer: the laws of a nation are the laws a nation gives itself.

Question #2 What justifies laws?

Positive law answer: that someone with power made it into the law. Natural law answer: that it matches what our use of reason and our insight into nature tells us should be the law.

Here, the positive law theorist can argue that the natural law theorist is also committed to his answer. Something only becomes a law when someone has power, and in a sense this is the justification of any law. E.g.,

Positive law justification: "why should I do what you're telling me?" "Because I will take away your TV time, and I can because I set the rules around here."

vs.

Natural law justification: "Why should I do what you're telling me?" "Because the concept of man is a rational animal who needs to develop his or her faculties of reason and the body in order to reach a state of true fulfillment. Oh, and if your reason doesn't make that sufficiently clear to you yet, then well, I will discipline you by taking away your TV time until you attain the state where you can manage your virtues and vices in accordance with reason which is a part of your nature not yet fully realized."

I think these two questions enable us to better address the claim of the legal positivists.

I'd suggest instead of "all law is positive law" that what we wind up with is:

if the justification for law is the power to enforce it, then all laws are positive laws.

This contains a condition, the positive lawyer accepts but the natural lawyer does not.

and

that something is the law means that it is a positive law (in the mode of how it became law)

which is trivial.

Question #3: So, why then do legal positivists not care that there are alternate answers to legal questions?

So there's natural law and rational constructivism and divine command theory, but these are not at all convincing for the legal positivists. The reason is that legal positivism is generally joined to a commitment that all we do when we work with language is shuffle terms.

Ergo, the attempts to show that there's any real sense of justification (i.e. a metaphysical reason why law ought to bear a certain character) is just meaningless jibber jabber masquerading as rationality. Similarly, any attempt to find a basis for law in nature is again just cultural biases and other things masquerading as something more than they are.

In other words, legal positivism is generally committed deeply to an anti-realist reductionism, and consequently it doesn't consider what others do to be what they purport to do but rather only purporting simpliciter.

(if the OP wants to clarify that he wants a specific explanation of the exact way Holmes argues this -- then this answer isn't fitting. I'm answering what I take OP to be asking -- which is why does Holmes not think there's any other kind of law?)

tl;dr - the degree to which you find Holmes et al.'s claim that all law is positive law convincing is basically linear with the degree to which you find the underlying claim that we're not connecting with reality with our "reasoning" convincing. The less convincing you find that claim, the less convincing positivism. The more convincing the claim, the more obvious positivism about law becomes.

Answer 2

One reason is general philosophical assumptions (best dealt with on Philosophy SE). Broadly speaking, classical Aristotelian principles have largely been replaced with Kantian thinking, and legal positivism is one version of that way of seeing man in relation to society. Ultimately it comes down to the difference between the Aristotelian "what objectively exists" view versus the Kantian "how it appears" view, the latter view now being (apparently) more widely accepted. A related matter is that it has been difficult (not impossible) to avoid invocation of a divine source for law. Appeals to "the common good" run into analogous problems with objectively defining and measuring that standard for identifying law. There are vastly fewer problems with determining whether a city has enacted an ordinance.

A second reason is what is known as the separability thesis (a feature of legal positivism), that legal validity is distinct from moral validity, whereas natural law holds that legal and moral validity are largely to entirely the same (the "overlap" thesis). If you believe that a man's right to his property is absolute, then it follows that a law allowing taxation is "bad law" from the moral perspective: there is no natural law to the effect that you must surrender 38% of your income to the sovereign. But it is absurd to think that there is no law requiring you to pay income tax. On the other hand, there really is no law requiring you to slaughter a goat during the Vernal Equinox. Legal positivism says, simply, that there does happen to be a law requiring payment of taxes (currently, in the US), and there is no Equinox Goat Slaughtering law (currently, in the US). Either fact could change if The Lawmakers say so, and that is obviously correct.

This is a reasonable article on legal positivism, as is this, and for natural law, this article and this are useful.

Answer 3

They don't. at least not in as far reaching a sense as you've written. The treaty of Natural Law that people like Wendell Holmes opposed does not cover all the laws of nature, like the laws of physics. It only covers laws of Human Nature like rights and morality. Holmes rejects the existence of such laws in the constitution largely from an historical analysis of the degree to which they have evolved and concludes that had they been some omnipresent force they would have remained relatively stable over time. For Holmes they had not been sufficiently stable to convince him that any such force could be present. From The Path of Law

"The danger of which I speak is not the admission that the principles governing other phenomena also govern the law, but the notion that a given system, ours, for instance, can be worked out like mathematics from some general axioms of conduct. This is the natural error of the schools, but it is not confined to them. I once heard a very eminent judge say that he never let a decision go until he was absolutely sure that it was right. So judicial dissent often is blamed, as if it meant simply that one side or the other were not doing their sums right, and if they would take more trouble, agreement inevitably would come."

And From the Common Law

"...in order for us to know what it is [law], we must know what it has been and what it tends to become"