Although the early Kant showed a complete willingness to dissent from many important aspects of the Wolffian orthodoxy of the time, Kant continued to take for granted the basic rationalist assumption that metaphysical cognition was possible. In a retrospective remark from the Prolegomena to Any Future Metaphysics (1783), Kant says that his faith in this rationalist assumption was shaken by David Hume (1711-1776), whose skepticism regarding the possibility of knowledge of causal necessary connections awoke Kant from his “dogmatic slumber” (4:260). Hume argued that we can never have knowledge of necessary connections between causes and effects because such knowledge can neither be given through the senses, nor derived a priori as conceptual truths. Kant realized that Hume’s problem was a serious one because his skepticism about knowledge of the necessity of the connection between cause and effect generalized to all metaphysical knowledge pertaining to necessity, not just causation specifically. For instance, there is the question why mathematical truths necessarily hold true in the natural world, or the question whether we can know that a being (God) exists necessarily.
The issue that vexed Kant was central to what 20th-century scholars called "the philosophy of mind ". The flowering of the natural sciences had led to an understanding of how data reaches the brain. Sunlight falling on an object is reflected from its surface in a way that maps the surface features (color, texture, etc.). The reflected light reaches the human eye, passes through the cornea, is focused by the lens onto the retina where it forms an image similar to that formed by light passing through a pinhole into a camera obscura . The retinal cells send impulses through the optic nerve and then they form a mapping in the brain of the visual features of the object. The interior mapping is not the exterior object, and our belief that there is a meaningful relationship between the object and the mapping in the brain depends on a chain of reasoning that is not fully grounded. But the uncertainty aroused by these considerations, by optical illusions, misperceptions, delusions, etc., are not the end of the problems.
The democratic peace proposition has been lurking in Western thought for millennia, as Weart 1998 shows, but Kant 1991 provides its first modern formulation. The idea that global democracy would provide a solid foundation for global peace was restated in 1917 by Woodrow Wilson as a justification for American entry into World War I and then as part of his vision for a new world order. Modern political science first observed the dyadic democratic peace—that democracies tend not to fight each other—in the 1970s. The observation enjoyed greater attention in the 1980s in particular in two pathbreaking 1983 essays by Michael Doyle, reprinted in Doyle 2011 . It received fuller theoretical and empirical attention in the 1990s. Fukuyama 1992 , a famous argument that humanity had reached “the end of history,” incorporates the democratic peace proposition. Other scholars sought to develop the theory and push forward more advanced research designs in works such as Russett 1993 ; Ray 1995 ; and Rousseau, et al. 1996 . In the 2000s, proponents of the democratic peace responded to their critics and embedded the democratic peace in a broader Kantian peace ( Russett and Oneal 2001 ).