Most libertarians would point to the principle that one should never initiate violence and use force only for self defense as the core axiom for liberty—the axiom that I call NVP, the Non-Violence Principle. Here I write to propose a second axiom, based on some recent conversation with my fellow Libertarian Party members.
I know one LP member who opposes big government except for public education to give poor kids a leg up. I know another who opposes big government except for Social Security for retirees, on the belief it isn't fair to deny someone benefits they paid into for decades. I know a third who opposes big government except for welfare and food stamps for the very poor. This person is convinced that a Marxist revolution will happen if all welfare is cut. I have also heard a Libertarian talk about the "real" pain and suffering of the poor that is alleviated by welfare, as if the pain caused by big government is not equally real.
These people, and I now suspect most Americans, understand libertarian economics, but they think that theft (in the form of taxation) is justified if it is for a good, worthy cause. Each person has his own pet cause that he wants government to fund, even while wanting taxes cut to pay for anything else.
I propose a new axiom: that the ends never justify the means. I term this the Anti-Marxist Axiom. If you believe this, then theft is never justified, even for the noblest purpose, and even if the rich have more money than they need. My justification for this axiom is moral, not pragmatic, and, in a weird way, Kantian. Kant's signature contribution to ethics is the theory of the Categorical Imperative, which I interpret to mean that, for something to be good, it must be right at all times and places universally. If there is an exception to an ethical (or political) principle then it was not rational or true, it was merely an expediency of the moment. To be a coherent theory, libertarianism needs the Anti-Marxist Axiom, otherwise it is just a rule of thumb to be compromised or abandoned when someone feels justified in doing so. If you use evil means to achieve good ends, logically the result will not be ethical, because you conceded to evil in order to achieve your goal.
If you want to fund a good cause with taxes then you conceded the validity of statism. If you accept that people make and earn money, and thereby morally deserve to own wealth, and then say that you can take someone's money away from them to spend as you see fit, even for a good cause, you have conceded and condoned widespread systemic theft. It should not then surprise you that a bunch of crooks, literally thieves, actual criminals, will run for office to acquire this opportunity and then will raise taxes on you to pay for evil things while spouting all sorts of virtuous good causes to justify it. There is a saying "power corrupts, and power attracts the corruptible." (Attributed to Frank Herbert.) I can say something similar: theft attracts criminals. This is a necessary and sufficient explanation for why big government is evil and will always become evil even if it begins as good.
Absent this axiom, you will find good cause after good cause, requiring tax raise after tax raise, and more and more theft to pay for your virtuous plans, until, from a libertarian starting point, you inevitably collapse into socialism. Either you have a universal, absolute axiom, or you face a very realistic slippery slope--even if sliding down it takes a nation 200 years.
Libertarians should consider abandoning their pet causes and commit to the Anti-Marx Axiom, to protect the purity of our principles. Libertarianism as a political theory needs an axiom, a self-evident principle to justify itself. If it does not have a principle then it is not a theory, it would be a mere pragmatic movement, or merely a feeling that government is bad. NVP is a good axiom, but many libertarians feel justified in making exceptions. The axiom that the ends never justify the means says there are no exceptions. If people want compromise, let them vote for the establishment. If they want principled politics, then they should vote for us. But how can we be a party of principles if we don't know what our core principle is?