Immanuel Kant (22 April 1724 – 12 February 1804) was the most influential of all the early modern philosophers. His writings remain to this day essential reading in aesthetics, ethics, political philosophy, philosophy of mathematics, epistemology, and metaphysics. In the nineteenth century he was a central influence on idealists like Hegel as well as to logicians and mathematicians like Bolzano, Weierstrass, and Frege. In the first half of the 20th century his mature writings set much of the agenda of the logical positivist (later logical empiricist) movement in philosophy of science and epistemology, in particular the writings of Carnap.
In his greatest work, the Critique of Pure Reason (1783), Kant argued that certain concepts (and in particular spatio-temporal concepts) were innate to the structure of human sensibility and hence had an apriori status. In this way he sought to explain how the laws of Euclidean geometry could be universal and exact, and known to be universal and exact, and yet to apply to physical bodies. All genuine (synthetic, as opposed to analytic) knowledge required the combination of sensibility (or ‘intuition’) and reason. The interplay of the two, in accordance with definite organising principles (or ‘categories of the understanding’), was alone capable of providing substantive knowledge. But according to Kant, the object of knowledge, conditioned in this way, was not the thing in itself, reality as it is, independent of how or what we know about it, as hitherto conceived by metaphysicians. Only objects as ‘appearances’ could be known; scientific knowledge concerns only things as appearances. But in requiring that the spatiotemporal structure of such objects conform to human sensibility, Kant’s position quickly lurched towards idealism. To his discomfort he was often compared to Bishop Berkeley.
Kant’s writings fall into two distinct periods: his early writings, in which he attempted to mediate the disputes of the Newtonians and Leibnizians, and to articulate Wolf’s metaphysical system (which broadly followed Leibniz) to Newtonian science; and his mature writings following, after more than ten years of silence, his Inaugural Dissertation of 1770. The key break with his earlier work was undoubtedly triggered by his argument from incongruent counterparts (handed objects) in Directions of Space (1769). In the Dissertation, he concluded that not only our grasp of spatial orientation, but of spatial and temporal concepts more generally, was intuitive rather than intellectual, and thus that space and time were mere intuitions, not substances, nor grounded on spatial and temporal relations of things independent of ourselves. Like Leibniz, Kant’s reliance on broadly Aristotelian logic came to be seen as a fundamental failing of his system.
In the Critique he argued that key questions in cosmology, such as whether the universe is finite or infinite in size, or had a beginning, were strictly meaningless. In his early period Kant had speculated on the nature of the Milky Way; he was the first to propose that it was the inner core of disc of stars, seen from within. He also proposed that nebulae, in particular the Andromeda nebulae, was a system of stars similar to and external to the Milky Way – an ‘island universe’ --