I mentioned previously that the modern conception of rights is bankrupt and dishonest -- that those who profess a belief in "individual" or "human" or "natural" "rights" while denying metaphysics are either ignorant and confused or evil liars. I pointed out that the modern, antimetaphysical conception of rights are really not rights at all but merely conventions which are subject to change, hence why leftism is constantly inventing new rights and discarding old ones.
Libertarianism, unfortunately, is an inadequate antidote to this, because its metaphysical basis for asserting rights is only marginally more coherent (and still false). When I speak of libertarianism here, I am not speaking of pragmatic or utilitarian libertarianism, the sort most frequently asserted by conservatives in the romantic tradition of Burke, which holds that small and unintrusive government is preferable because it maximizes value and because the alternatives are unworkable and therefore undesirable. Rather, I am speaking of what could be called deontological libertarianism, which asserts a framework of rights and imposes on individuals, organizations, and states a duty to respect them; it is this belief that characterizes most of the people who identify today as some form of libertarian, especially those of the big-L variety.
As I'm given to understand it, modern deontological libertarianism relies on two principles that are held to be axiomatic. The first is commonly called the self-ownership principle, the second the nonaggression principle. The former holds (obviously) that man has ownership of himself, and that by virtue of this self-ownership he may assert sovereignty over property because he endows it with some aspect of himself. The latter simply holds that aggression (i.e., initiated, unprovoked use of force) is illegitimate.
These two principles (usually in combination to some extent) are held to yield up an system of rights. For instance, a (self-owning) man may chop down a tree and use the wood to build a boat; because he endowed the wood with his labor, the boat belongs to him because no others have a right to it. To say others had a right to it would imply that they have a right to his labor, which he would have an obligation to provide; but he, being self-owning, has no such right. (This assumes that he owned the tree in the first place, or at least that the tree was unowned, but more on that in a minute). The nonaggression principle (which is not, strictly speaking, axiomatic, but is usually regarded as if it were) suggests that aggression which seeks to deprive individuals of these rights is illegitimate, typically because it violates the self-ownership principle.
Now, of course, an individual may to some extent yield his rights to others. He may consensually contract out his labor to another. Thus, a man may agree to chop down his neighbor's tree and use the wood to build a boat for him in exchange for handsome payment does not have a right to the boat; the boat is his neighbor's. A kind of mutually-beneficial exchange of rights has occurred here: the neighbor has (temporarily) given up his tree and the man his labor so that the former might get a boat and the latter might get paid.
Consent therefore works out to be the glue that holds deontological libertarian rights theory together. The man cannot build a boat and then assert ownership over it because he consented not to have any stake of ownership in it in the first place.
So consent winds up being an unarticulated axiom here: that you have a duty to do that to which you consent to do (within reason).
But where does this duty come from? I did not consent that my consent should be binding. To say that it is binding on me is therefore to impose upon me an external constraint to which I never consented -- which should be considered a violation of my self-ownership. You can reconcile the two only by acknowledging that there are limits on self-ownership, or by acknowledging that there are higher duties than those which are imposed upon you by choice. Either acknowledgment is fatal to the theory. If self-ownership is limited, it cannot be universal, much less axiomatic. If there are higher duties than those imposed merely by choice, than the entire theory is contingent on assumptions furnished only by natural law, which cannot approve of libertarian rights theory.
A second problem presents itself with the self-ownership axiom. Asserting that one owns oneself assumes that there is a right to own things in the first place, which is a right which self-0wnership is used to assert. Self-0wnership, then, becomes circular reasoning: my right to own property derives from my ownership of my self; but my self-ownership derives from my right to own things in general. Here, the libertarian ethicist might be inclined to say that the principle is axiomatic, then one does not need to know where the right to self-ownership comes from to assert it, and that he is simply saying that a libertarian conception of rights follow logically from its assertion. The lattermost point may well be true, but this gives the game away: an axiom is an axiom because it is obviously true; and whether or not there is anything such thing as a right to own anything is precisely what's at issue here. The principle therefore gives us no basis for deciding whether or not the libertarian conception of rights is true or accurate at all.
It's worth noting that libertarianism itself necessarily imposes unconsented-to restrictions on individual action, anyway, so that my being the biggest and strongest man on the block may grant me the capacity but not the right to pummel, pillage, and rape others (hence, nonaggression). But why so? I never consented that my rights should be restricted in this way (I certainly didn't and don't consent to this self-ownership principle), yet consent must be supreme if we are to be able to reconcile self-ownership with the functionality of the society we see around us. The libertarian ethicist may reply, "Yes, but you must agree to this restriction so that a liberty-promoting framework may be maximized." But this also gives the deontological game away, since it acknowledges that its conception of "rights" is really just a consequentialist convention and that "rights" themselves are, at least some of the time, subordinate to the good of social value-maximization.
The libertarian conception of "rights" isn't really much more tenable than the modern leftist one. It must necessarily degenerate into either leftist consequentialism (by abandoning the universality and absolutism of rights in order to enable itself to function) or sin (by acknowledging the supremacy of natural law but refusing to live in accordance with it). In either event, it is certainly not reasonable, much less a valid or desirable alternative. Which stands to reason, since libertarianism itself an earlier innovation of modernity.