Conifold is right - we need to look further back than the Enlightenment. No historical phenomenon can be given a fixed date of origin but the Enlightenment as Popper would have understood it was predominantly an 18th-century movement. Popper speaks of 'three hundred years', which takes us back (from 1945) to the 17th, not the 18th, century. Also if Popper was referring to the Enlightenment, why doesn't he call it that ? The label is and was familiar enough.
I think a better, more plausible candidate is the so-called scientific revolution of the 17th century. We can and historians do argue about how far it was a revolution and when and where to locate its growth-point. But I nominate Newton (1642-1727) as an agent in a scientific revolution of a quite definite kind - mathematical and empirical.
No reader of Newton's Principia (1687) can fail to see the central picture that emerges from it. Combining both reason in the form of mathematical equations and a newtwork of definitions, axioms and theorems, and statements of empirical relationships, Newton presented a view of the world as a vast mathematical system, yielding accurately verifiable observations, and in principle enabling us to control, manipulate and predict physical phenomena. A sense of power over nature was generated.
The link between Newtonian physics and what Popper is talking about - the 'attempt to build up an open society which rejects the absolute authority to preserve, to develop, and to establish traditions, old or new, that measure up to their standards of freedom, of humaneness, and of rational criticism' - was mediated not least by John Locke (1632-1704), a friend of 'the incomparable Mr Newton'.
Locke also is taken with the power of reason even if in his hands reason acquires a different reading :
... reason, in Locke, was capable of yet further interpretations when located in man's perception of his political state on earth. For here, according to Locke, man, as an individual, had attached to himself certain "natural rights", inalienable and neither capable nor needful of further explanation and defence, such as in earlier centuries had been expressed in terms of the divine right of kings. Man possessed these rights when "in a state of Nature"; and "the state of Nature has a law of Nature to govern it". And what is this "State of Nature"? Locke is quite clear : "Men living together according to reason ... is properly the state of Nature." (Gerd Buchdahl, The Image of Newton and Locke in the Age of Reason, London & NY : Sheed & Ward, 1961 : 23; Locke, Second Treatise of Civil Government, II.2 & 3.)
Here were forged powerful intellectual weapons, to be used on later occasions, by the American revolutionaries, or the universal emancipationist movements of the French Revolution. (Buchdahl : 23.)
Of course, despite Locke's immense influence in Europe and especially France in the 18th century - the Age of Enlightenment - other figures such Montesquieu, Diderot and Voltaire transformed - reoriented - 'the movement' of which Locke was a pioneer.
I have two reservations about Buchdal. (a) It can hardly be the case for Locke that natural rights are 'neither capable nor needful of further explanation and defence'. They are according to Locke objective moral attributes possessed by human beings and their existence is deducible from the Law of Nature. Secondly, (b) a rational politics independent of tradition or divine right is to my mind, or at least arguably, present in Hobbes' Leviathan (1651). But Hobbes himself, of course, was powerfully influenced by the 'new science'. Neither point affects the substance of my argument, however. Locke's significance is unaltered.
The following objection has been put :
Wikipedia lists 1650 and 1700 as alternate start dates for the enlightenment, and History.com lists 1685. The time period is right. I am a fan of Locke's enlightenment thinking, and am uncomfortable with including Naziism and Communism as byproducts, but Popper was a notably big thinker, and I am just tryng to follow his lead here. The science revolution did not have the moral component that moralizing ideologies offer, so tying Comunism and Naziism to science seems even more of a stretch.
I make two points :
Dating the Enlightenment to the 18th century was still maintained by one of its principal modern scholars, Peter Gay: '... the narrow Enlightenment of the philosophes was embedded in a wider, more comprehensive atmosphere, the atmosphere of the eighteenth century, which may be called, without distortion, the Age of the Enlightenment' (P. Gay, The Enlightenment : An Interpretation - The Science of Freedom, NY & London : W.W. Norton, 1977 : Preface, x.)
There are decisive reasons to revise Gay's chronology. I endorse
Jonathan Israel's back-dating of the Enlightenment :
**[Israel] ... offered an ambitious revision of the chronologic and
geographic boundaries of Gay's Enlightenment - a revision which was indispensable, Israel argued, to grasp the modernizing role of the Enlightenment. In
his first volume, Radical Enlightenment: philosophy and the making of modernity,
Israel turned his attention away from the eighteenth-century French heartland
mapped by Gay. Instead, he focused on developments in the Dutch Republic of
the mid-seventeenth century. There, the Jewish-Dutch philosopher Baruch de
Spinoza was cooking up a heady mixture of atheism and democratic republicanism. Spinoza's writings in turn inspired a radical movement that rapidly
fanned out from the Dutch Republic to other European countries such as
England, Italy, Germany, and France. This movement, Israel argued, constituted a 'radical Enlightenment', whose core ideas were far more modern and
forward-looking than anything imagined by Gay's philosophes. Indeed, major
philosophes such as Voltaire and Montesquieu, being deists rather than atheists and defenders of the status quo rather than revolutionaries, were proponents of
a very different, 'moderate', or conservative Enlightenment, which postdated
the radical, Spinozist Enlightenment and had mainly emerged in reaction
against it. (ANNELIEN de DIJN, 'THE POLITICS OF ENLIGHTENMENT: FROM PETER GAY TO JONATHAN ISRAEL, The Historical Journal, Vol. 55, No. 3 (SEPTEMBER 2012), pp. 785-805 : 797-8.)
Re-dating is not the point
Popper wrote in 1945. The question is not, how do we now date the Enlightenment but rather, how would Popper have been likely to date the Enlightenment in 1945 ? He didn't have the advantages of modern revisionism and was no historian. It is almost certain that he held the then prevailing view of the Enlightenment as a predominantly 18th-century phenomenon :
For centuries, Europeans lived under the combined tyranny of priest and king.
The Renaissance and Reformation dented the power of this hybrid monster
over men's minds. But around the turn of the eighteenth century, a much more
fundamental challenge to the status quo emerged. A new generation of men (for this is a story without women) stood up and cast off the shackles of
superstition and authority in the name of reason. Taking their cue from
Holland and especially England, the most liberal European nations, they waged
a war for religious and political freedom. The generals of this war, a little flock
of self-styled 'philosophes', resided in France, but troops were enlisted
throughout the whole of Europe. Displaying great courage, wit, and
perseverance, they managed to gain the upper hand against the forces of
darkness. By the end of the eighteenth century, a mental revolution had been
achieved. The events of 1776 and 1789 were the outcome of this intellectual
sea-change. The modern, liberal democracies they created put the philosophes'
programme into practice.
We all know this story. It is, of course, the textbook version of Enlightenment
history. (ANNELIEN de DIJN, 'THE POLITICS OF ENLIGHTENMENT: FROM PETER GAY TO JONATHAN ISRAEL, The Historical Journal, Vol. 55, No. 3 (SEPTEMBER 2012), pp. 785-805: 785-6.)
There is no reason to suppose that Popper did not subscribe to this orthodoxy in 1945. If so, then 300 years back would have landed him, not in the Enlightenment, according to the then regnant orthodoxy, but in the so-called scientific revolution of the 17th century. This was and remains my claim.
- My answer does not tie 'Communism and Nazism to science'. My suggestion was only that the Enlightenment was informed by the so-called scientific revolution of the 17th century, which cleared the way for or reinforced the idea that human society is open to the same control, manipulation and prediction as physical phenomena. This idea, whatever its cogency or otherwise, is morally neutral : it produced the relative beneficence of the American Revolution and the horrors (the 'appalling destructiveness') of the Reign of Terror in the French Revolution. Popper believed that he had witnessed its 'appalling destructiveness' in the Second World War. Its powers for such destructiveness may yet be conquered, he surmised and hoped, just as they were deflected - to offer him an example - in the American Revolution.