Lysander Spooner, like Josiah Warren, was one of those American geniuses who developed his thought pretty much independently of others. It was only later in life that he developed close associations with other anarchists, such as Benjamin Tucker, William B. Greene, and other "Boston anarchists." In the classical liberal tradition of Locke, Jefferson, and Paine, Spooner championed natural law as the basis for his political theories.
Spooner was one of those rare persons who became more and more radical as he got older. In his younger days, he was a minarchist, and putting emphasis on the US Constitution and its precedents, the Magna Carta and Common Law. Since he was trained as a lawyer, this is not surprising. However, even in his constitutionist youth, his ultimate appeal was to natural law. Thus, he interpreted the Constitution as a man-made (and therefore flawed) attempt to describe natural law, and rejected the parts of the Constitution that violated this law. The very term "law," according to Spooner, referred not to man-made, decreed, or legislated law, but to higher natural law.
What then is LAW? That law, I mean, which, and which only, judicial tribunals are morally bound, under all circumstances, to declare and sustain? In answering this question, I shall attempt to show that law is an intelligible principle of right, necessarily resulting from the nature of man; and not an arbitrary rule, that can be established by mere will, numbers or power. To determine whether this proposition be correct, we must look at the general signification of the term law.
The true and general meaning of it, is that natural, permanent, unalterable principle, which governs any particular thing or class of things. The principle is strictly a natural one; and the term applies to every natural principle, whether mental, moral or physical. Thus we speak of the laws of mind; meaning thereby those natural, universal and necessary principles, according to which mind acts, or by which it is governed. We speak too of the moral law; which is merely an universal principle of moral obligation, that arises out of the nature of men, and their relations to each other, and to other things and is consequently as unalterable at the nature of men. And it is solely because it is unalterable in its nature, and universal in its application, that it is denominated law. If it were changeable, partial or arbitrary, it would be no law. - Lysander Spooner, The Unconstitutionality of Slavery (1845, 1860)