An English historian A. J. Carlyle (1861–1943) notes in A History of Medieval Political Theory in the West, vol. 1: “There is no change in political theory so startling in its completeness as the change from the theory of Aristotle to the later philosophical view represented by Cicero and Seneca.... We think that this cannot be better exemplified than with regard to the theory of the equality of human nature." Charles H. McIlwain likewise observes in his writing, The Growth of Political Thought in the West: From the Greeks to the End of the Middle Ages that "the idea of the equality of men is the profoundest contribution of the Stoics to political thought" and that "its greatest influence is in the changed conception of law that in part resulted from it.
At the time of the Roman Stoic school (ca. 100 BC to 100 AD) the idea of a natural equality of human beings found expression in the works of Cicero and Seneca. They made it an integral part of their understanding of a universally valid natural law.
Both Aristotle and his student Plato had a major influence on the Stoics; yet, it can be argued that it was the Stoics who first formulated the concept of “natural rights” or “unalienable rights”. The Stoics contended that there is something about the rational power of humanity that confers special powers that are generative of rights and it is by virtue of the powers of abstraction and comprehension that we have liberties at all. Rational power, according to Stoic teaching, is the dominant faculty in healthy human life.
Plato contended that for something to exist it must be capable of acting or of being acted upon. This led most to conclude that existence was confined solely to tangible, physical objects. Stoics accepted Plato’s work as foundational, but also understood that non-physical entities must be included in any valid account of reality. Space and time, for example, are not physical entities but must be part of any valid conception of physical reality itself. Stoicism retained that reality is governed by law and that human affairs must approximate and be obedient to those same principles of lawfulness.
According to Plato we live in an orderly universe. Stoics also recognized nature as an ordered realm. They viewed it as governed by immutable laws that collectively establish what is “right” for the entities that compose it. They taught that the essential nature of law or morals is rational, proportionate, and unprejudiced. Natural law, or the law of nature is a system of law that is determined by nature, and as such is universal.
Aristotle emphasized the distinction between "nature" on the one hand and "law", "custom", or "convention" on the other. What the law commanded varied from place to place, but what was "by nature" should be the same everywhere. A "law of nature" would therefore have been something of a non sequitur to him. Neither concept would follow the other. There is evidence, however, of Aristotle having thought there was a natural law and this comes from his work titled Rhetoric. Here Aristotle notes that, aside from the "particular" laws that each people has set up for itself, there is a "common" law that is according to nature. Specifically, Aristotle quotes Sophocles and Empedocles: Universal law is the law of Nature. For there really is, as everyone to some extent divines, a natural justice and injustice that is binding on all men, even on those who have no association or covenant with each other.
Although the term “natural law” is often used interchangeably with “common law”, the two are distinct in that natural law is a view that certain rights or values are inherent in or universally applicable by virtue of human nature, while common law is the legal tradition whereby certain rights or values are legally applicable by virtue of judicial recognition or articulation.
The rise of natural law as a universal system coincided with the rise of large empires and kingdoms in the Greek world. The “natural” or “higher” law suggested by Aristotle to which one could appeal was strictly and emphatically natural and based in human reason. It gave no consideration to divine origin or legislation. The Stoic natural law was indifferent to either the divine or natural source of the law. The Stoics asserted the existence of a rational and purposeful order to the universe. The means by which a rational being lived in accordance with this order was the natural law; this meant behavior that accorded with virtue, which was considered natural.
On the question of good and evil, the Stoics are defenders of a form of rationalism that once again resists supernatural or mystical explanations. They clearly held, what we really mean by “evil” is a departure from virtue and from the natural order of things. For them, it was purely rational. We are rational beings living in a rational and purposefully ordered universe.
What is it in human life that most characteristically defines the counsels of reason or rationality? That is more easily answered from the negative interrogative. So, what is it that most regularly defies the counsels of reason? The answer to that is quite simple. Emotion and passion are what most contradict reason or rationality. What most defies reason or rationality is anything in which we have so vested an emotional interest that we will do things on behalf of that, no matter what the counsels of reason might recommend.
To the Stoics, the equality of human beings was both good and rational. It was in keeping with virtue and the natural laws of a rational and purposefully ordered universe to which both the universe and its inhabitants were subject. In fact, they believed it was the rational power of humanity itself that conferred special powers generative of the rights we still claim and cherish.
Does this place the writers of the Declaration of Independence among the ranks of the Stoics? It certainly does not. Like the Stoics, our forefathers accepted as foundational what was good among the works of their predecessors, but they built on top of that foundation. Both the Stoics and our forefathers held the equality of all men to be self-evident, but they disagreed on the origin of that equality and the associated unalienable rights. While the Stoics did not oppose the idea of God or a Creator, they were indifferent to it. The writers of our Declaration of Independence clearly believed in a Creator and attributed to that Creator the rights we celebrate to this day. While their specific religious practices differed, the founding fathers of the United States and the writers of the Declaration of Independence believed in the God of the Jews, of Islam and of Christianity. This God befriended mankind as a father does his children. Under this God, all people are brothers and sisters, no matter their country or social standing. No one is of greater worth than any other. According to our forefathers, this is the universal law of our nature that grants us unalienable rights and upon which our independence was declared.