The First Treatise is an extended attack on Sir Robert Filmer's Patriarcha. Locke's argument proceeds along two lines: first, he undercuts the Scriptural support that Filmer had offered for his thesis, and second he argues that the acceptance of Filmer's thesis can lead only to slavery (and absurdity). Locke chose Filmer as his target, he says, because of his reputation and because he "carried this Argument [jure divino] farthest, and is supposed to have brought it to perfection" (1st Tr., § 5).
Filmer's text presented an argument for a divinely ordained, hereditary, absolute monarchy. According to Filmer, the Biblical Adam in his role as father possessed unlimited power over his children and this authority passed down through the generations. Locke attacks this on several grounds. Accepting that fatherhood grants authority, he argues, it would do so only by the act of begetting, and so cannot be transmitted to one's children because only God can create life. Nor is the power of a father over his children absolute, as Filmer would have it; Locke points to the joint power parents share over their children referred to in the Bible. In the Second Treatise Locke returns to a discussion of parental power. (Both of these discussions have drawn the interest of modern feminists such as Carole Pateman.)
Filmer also suggested that Adam's absolute authority came from his ownership over all the world. To this, Locke responds that the world was originally held in common (a theme that will return in the Second Treatise). But, even if it were not, he argues, God's grant to Adam covered only the land and brute animals, not human beings. Nor could Adam, or his heir, leverage this grant to enslave mankind, for the law of nature forbids reducing one's fellows to a state of desperation, if one possesses a sufficient surplus to maintain oneself securely. And even if this charity were not commanded by reason, Locke continues, such a strategy for gaining dominion would prove only that the foundation of government lies in consent.
Locke intimates in the First Treatise that the doctrine of divine right of kings (jure divino) will eventually be the downfall of all governments. In his final chapter he asks, "Who heir?" If Filmer is correct, there should be only one rightful king in all the world—the heir of Adam. But since it is impossible to discover the true heir of Adam, no government, under Filmer's principles, can require that its members obey its rulers. Filmer must therefore say that men are duty-bound to obey their present rulers. Locke writes:
I think he is the first Politician, who, pretending to settle Government upon its true Basis, and to establish the Thrones of lawful Princes, ever told the World, That he was properly a King, whose Manner of Government was by Supreme Power, by what Means soever he obtained it; which in plain English is to say, that Regal and Supreme Power is properly and truly his, who can by any Means seize upon it; and if this be, to be properly a King, I wonder how he came to think of, or where he will find, an Usurper. (1st Tr., § 79)
Locke ends the First Treatise by examining the history told in the Bible and the history of the world since then; he concludes that there is no evidence to support Filmer's hypothesis. According to Locke, no king has ever claimed that his authority rested upon his being the heir of Adam. It is Filmer, Locke alleges, who is the innovator in politics, not those who assert the natural equality and freedom of man.