Incongruent counterparts are objects that are perfectly similar except
for being mirror images of each other, such as left and right human
hands. Immanuel Kant was the first great thinker to point out the
philosophical significance of such objects. He called them "counter-
parts" because they are similar in nearly every way, "incongruent"
because, despite their similarity, one could never be put in the place
of the other. Three important discussions of incongruent counterparts
occur in Kant's writings. The first is an article published in 1768, 'On
the First Ground of the Distinction of Regions in Space', in which Kant
con- tended that incongruent counterparts furnish a refutation of
Leibniz's relational theory of space and a proof of Newton's rival
theory of absolute space. The second is a section of his Inaugural
Dissertation, published two years later in 1770, in which he cited
incongruent counterparts as showing that our knowledge of space must
rest on intuitions. The third is a section of the Prolegomena to Any
Future Metaphysics of 1783, in which he cited incongruent counterparts
as a paradox resolvable only by his own theory of space as
mind-dependent. A fourth mention in the Metaphysical Foundations of
Natural Science of 1786 briefly repeats the Prolegomena point.
Curiously, there is no mention of incongruent counterparts in either of
the editions (1781 and 1787) of Kant's magnum opus, the Critique of Pure
Reason.