302 Kant: A Biographyappear at this Easter book exhibition. His appendix against Garve did not
materialize; rather, he is said to have shortened the work. He seems to suf¬
fer from diarrhea, and I am worried that he will lose his reputation as an
author by writing too much."^113 Indeed, it is amazing how much Kant
wrote between the spring of 1784 and the fall of 1786. He produced not
only the Groundwork, five major essays, and three installments of his re¬
view of Herder's Ideas, but also two other essays, a Preface to Jakob's Ex¬
amination of Mendelssohn's Morning Hours (1786), and another major work,
the Metaphysical Foundations of Natural Science. Even if he was drawing
on partial drafts from years of teaching and had an amanuensis who helped
him to prepare the final copy, the quantity and quality of his output is still
astounding.
On September 13, 1785, Kant could write that he had finished a book
during the summer but that, since he had injured his hand, the manuscript
would have to stay on his desk until Easter.^114 The book indeed appeared
at the following Easter convention of book dealers in Leipzig. In the Sep¬
tember 13 letter he explained the purpose of the book as follows:Before I go on to the promised Metaphysics of Nature I first had to deal with some¬
thing that is really only a mere application of it, but which still presupposes an empir¬
ical concept, namely the basic metaphysical grounds of the doctrine of bodies and, in
an appendix, the doctrine of the soul.^115The doctrine of the soul was dropped. As Kant explained in his Preface,
empirical knowledge of the soul can never become scientific. "Mathematics
is inapplicable to the phenomena of the internal sense and their laws."^116
There is only as much science in a subject as there is mathematics. There¬
fore, the doctrine of the soul, which must be based on inner sense, cannot
form part of the Metaphysical Foundations of Natural Science.
Kant had to write this book because he believed that science required
apodictic certainty. Merely empirical certainty is not enough, but apodic-
tic certainty can only be a priori. Therefore, we have natural science "only
when the natural laws are cognized a priori," and this means that "all nat¬
ural science proper requires a pure part upon which the apodictic certainty
sought by reason can be based."^117 This pure part can come only from the
universal laws of thought, which are ultimately based on the categories.
Kant's approach was to consider what he took to be the central concept of
the book, that is, matter, under "all the four headings of the understand¬
ing," namely quantity, quality, relation, and modality.^118 Therefore, the
book has four chapters, concerning the metaphysical foundations of phoro-