There are different views about this question. Some scholars see the 18th-century German philosopher Immanuel Kant as one of the central Enlightenment thinkers, while others claim that in him we can already see the shift away from the self-confident optimism about the power of human reason that marked the Enlightenment age. However, any answer to this question depends on what we understand to be essentially characterising a representative Enlightenment thinker. In line with this very helpful overview of the period, I propose that there are two such leading characteristics:
- Sceptical questioning of any dogma and authority, and
- Confidence in human cognitive capabilities to achieve systematic and complete knowledge of nature.
I think Kant satisfies both criteria, but it is with regard to the 2nd point that his thought initiates a fundamental shift of focus for many later philosophers. In particular, because more mature Kant establishes that human reason has no access to anything beyond the phenomenal world (experience, appearance), no serious metaphysics of the future should try to find out (or even pretend to know) the ultimate nature of the world as it is in itself (noumenal world). He compares such an old style of metaphysics to astrology or fortune-telling.
However, he believes in the capability of reason to critically investigate and establish all the a priori concepts that enable our minds to understand and interpret our experiences of the world. For example, he thinks that causality is one such concept. We cannot even begin to understand our experience of the world without the concept of causality present in our minds. How would we comprehend sequences of observed events without the idea of cause and effect?
Thus, the focus of metaphysics in Kant is turned toward achieving systematic and complete knowledge of the nature of our minds. Today, we might call them our basic presuppositions or, as Justin L. Barrett refers to them in his book Cognitive Science, Religion, and Theology, non-reflective beliefs acquired through maturational natural cognition (e.g, such beliefs as ‘I’, ‘I am’, the belief that one cannot pass through physical objects such as walls, etc.). This then, for Kant, would be the task of any new, critical metaphysics:
“In order that, as science, it [metaphysics] may lay claim not merely to deceptive persuasion, but to insight and conviction, a Critique of Reason must exhibit in a complete system the whole stock of conceptions a priori, arranged according to their different sources… it must present a complete table of these conceptions, together with their analysis and all that can be deduced from them, but more especially the possibility of synthetic knowledge a priori by means of their deduction, the principles of its use, and finally, its boundaries. Thus criticism contains, and it alone contains, the whole plan well tested and approved, indeed all the means whereby metaphysics may be perfected as a science – by other ways and means this is impossible. The question now is not, however, how this business is possible, but only how we are to set about it… This much is certain: he who has once tried criticism will be sickened for ever of all the dogmatic trash he was compelled to content himself with before, because his Reason, requiring something, could find nothing better for its occupation.”Kant, Prolegomena, 1783, in Cottingham, ed., Western Philosophy: An Anthology, 2008, p.114