I’ve defended the Greek philosophers as providing a better foundation for classical liberalism than Kant does. But I might fairly be asked: why then are these Greek thinkers generally so much less liberal than Kant in their actual political proposals?
The answer, I think, lies in one crucial insight that Kant, along with most other modern political thinkers, has that the Greek thinkers (meaning here, once again, mainly Socrates, Plato, Aristotle, and the Stoics) tended to lack.
The aforementioned Greek philosophers hold that the right reason to act justly is a love of virtue for its own sake, not a strategic concern to elicit cooperation from one’s neighbors. Kant would agree (though he would quarrel with the Greeks’ understanding of what it means to love virtue “for its own sake”; they take it to mean loving virtue as an intrinsic component of happiness, whereas Kant would take it to mean loving virtue apart from any connection to happiness). For Kant, those who behave justly in order to win external rewards or avoid external punishments are acting merely in accordance with rather than from duty.
But these Greeks go further, in thinking that no sociopolitical order can function properly unless most of its citizens are in fact motivated by virtue. Hence the Greeks see the inculcation of virtue as a crucial task of political institutions, and this accounts for most of the illiberal features of the ideal states put forward in Plato’s Republic and Laws, as well as (to a lesser degree) in Aristotle’s Politics. (Zeno the Stoic’s lost political treatise, also titled Republic, might seem an exception, since it reportedly dispensed with all mechanisms of state coercion; but Zeno’s ideal republic was also supposed to be a community of the wise only, and so did not contemplate the uncoerced cooperation of ordinary people.)
I’ve stressed that by the Greek philosophers I means specifically Socrates, Plato, Aristotle, and the Stoics, because there was another group of Greek philosophers – the Sophists – who very much looked to strategic thinking as the basis for social order. (A partial exception is the Sophist Protagoras, who is portrayed in Plato’s dialogue of that name as arguing that we wouldn’t see as much social order as we do if most people were purely strategically motivated.) But the Sophists also tended to hold that strategic thinking was the best reason to be just.
Here Kant essentially splits the difference. He agrees with Socrates, Plato, Aristotle, and the Stoics that strategic motivations are the wrong reason to be just; but against them, and with the Sophists, he agrees on the importance of strategic motivations to the maintenance of social order. The distinctively modern insight that Kant shares with such thinkers as Thomas Hobbes, Bernard Mandeville, and Adam Smith is that the right incentive system can lead people to behave in a just manner.
“[T]he problem of setting up a state,” Kant tells us in Perpetual Peace, “can be solved even by a nation of devils … so long as they possess understanding.” The key to securing cooperation among the citizens is not to foster the “moral improvement of man”; rather, the “constitution must be so designed that, although the citizens are opposed to one another in their private attitudes, these opposing views may inhibit one another in such a way” as to incentivize cooperation.
Here Kant arguably overstates his case, making the opposite mistake from that of Plato et al. Quite possibly no constitution could secure cooperation for a literal nation of devils; Protagoras is probably right in thinking that some degree of virtuous motivation in the populace is necessary in order for society to function. Thomas Paine surely strikes the right balance in tracing “that order which reigns among mankind” to a combination of “mutual dependence and reciprocal interest” on the one hand and “a system of social affections” on the other.
But Kant, along with his fellow moderns, is right in seeing that, given the appropriate external incentives, the level of virtue needed for society to function is much lower than that needed to make the participants count as fully morally worthy.
The political authoritarianism of the ancient Greek philosophers has always been in tension with their ethical individualism. But the perceived need for the state to mold its citizens’ characters arguably stems, at least in part, from the assumption that near-universal virtue is a prerequisite for proper social order. Kant’s retreat from that assumption allows him to defend a much less intrusive state than those we find in Plato or Aristotle.
Nor is Kant’s understanding of incentive systems limited to those deliberately constructed, like the formal constitution of a state. Unsurprisingly given his enthusiasm for Scottish Enlightenment thinkers like Adam Smith, Kant also makes room in his political philosophy for non-constructed “spontaneous order” or “invisible-hand” mechanisms. For example, in his Universal History Kant identifies the apparently destructive character of human beings’ competitive impulses, their “unsocial sociability,” as, paradoxically, one of the chief engines of social progress.
To be sure, Kant’s understanding of spontaneous order has its limits. For one thing, as previously noted, he fails to see the possibility of law without the state. For another, he identifies legislative fiat as the sole possible origin of money: “In this way only (so it seems to me) could a certain merchandise have become a lawful means of exchange ….” Kant fails to anticipate the arguments of Austrian economists like Carl Menger and Ludwig von Mises showing how a common medium of exchange could arise spontaneously out of a barter economy without state involvement.
On the other hand, some Austrian economists have gone so far as to maintain that money can only arise in the bottom-up manner described by Menger and Mises, and could never be the product of state action. Murray Rothbard, for example, writes: “Money must develop out of a commodity with a previously existing purchasing power, such as gold and silver had. It cannot be created out of thin air by any sudden ‘social compact’ or edict of government.” Here Kant deserves credit for seeing that this simply isn’t true; as Kant points out, a ruler can easily cause a given item to have universal exchange value simply by choosing to “levy taxes on his subjects in this material.”
 See, for example, the exchange among Socrates, Glauocn, and Adeimantus in Book II of Plato’s Republic: http://oll.libertyfund.org/titles/767#lf0131-03_head_014
 For this distinction, see Immanuel Kant, Groundwork of the Metaphysics of Morals, 2nd ed., trans. Mary Gregor and Jens Timmermann (Cambridge: Cambridge University Press, 2012), pp. 13-14.
 Here the Epicureans stand somewhere between the two groups.
 Plato, Protagoras: http://oll.libertyfund.org/titles/111#Plato_0131-01_1252
 For elaboration on the Sophists’ position, see Roderick T. Long, “The Pros and Cons of Justice,” Libertarianism.org (1 June 2016): https://www.libertarianism.org/columns/ancient-greeces-legacy-liberty-pros-cons-justice
and “Justice as a Sometime Thing,” Libertarianism.org (7 June 2016): https://www.libertarianism.org/columns/ancient-greeces-legacy-liberty-justice-sometime-thing
 Immanuel Kant, Perpetual Peace, in H. S. Reiss, ed., and H. B. Nisbet, trans., Kant: Political Writings, 2nd ed. (Cambridge: Cambridge University Press, 1991), pp. 112-113.
 Thomas Paine, Rights of Man II.1: http://oll.libertyfund.org/titles/344#lf0548-02_label_163
 On this tension, see Roderick T. Long, “Forced to Rule: Atlas Shrugged as a Response to Plato’s Republic,” in Edward W. Younkins, ed., Ayn Rand’s Atlas Shrugged: A Philosophical and Literary Companion (New York: Routledge, 2016), pp. 90-97.
 Immanuel Kant, Universal History, in H. S. Reiss, ed., and H. B. Nisbet, trans., Kant: Political Writings, 2nd ed. (Cambridge: Cambridge University Press, 1991), p. 44; cf. Perpetual Peace, same volume, p. 108.
 Kant, Doctrine of Right, p. 105/288.
 Carl Menger, “On the Origin of Money,” Economic Journal 2 (1892), pp. 238-255; Ludwig von Mises, The Theory of Money and Credit, trans. J. E. Batson (Auburn AL: Mises Institute, 2009).
 Murray N. Rothbard, Man, Economy, and State: A Treatise on Economic Principles; with Power and Market: Government and the Economy: Scholar’s Edition (Auburn AL: Mises Institute, 2009), p. 275.
 Doctrine of Right, p. 105/288.