DEDICATION:

are generated; why some of them leave fomes,' and some propa—
gate themselves even from a distance; why some diseases are
contagious, though they are milder and more gentle, while others,
though much more acute and more virulent, are not at all con—
tagious; how contagion differs from poisons; and many other
questions of the same sort.

I suppose that they considered that they had done enough
when they had reduced the causes of all these phenomena to oc—
eult properties, as they call them. And not only have they de—
cided to accept this theory for themselves, but they have even
taught it to others. Contented with this, they have developed
very many philosophical theories, a proceeding which I have al—
ways thought unworthy of a true philosopher.

Novw there are three classes of causes; those that are very gen—
eral and very remote from things; those that are nearer and more
special; and finally those that are very near and very special. In
obscure and difficult matters, to arrive at the special and nearest
causes is surely the affair of God or of the divine; to rest content
with the most general causes is a sign of a lazy and boorish mind;
but to investigate the intermediate causes, and to try to arrive
at the special, so far as & human being may, is surely the task of
a philosopher. / Vou will perhaps be surprised that I have under—
taken to write about these two subjects, one of which has been
left almost untouched by our ancestors, while the other is ad—
mitted by all to be beyond the reach of human knowledge. Cer—
tainly, I too admit that the sympathies of things and the nature
of contagions are very difficult and arduous themes to treat. I
will even admit that he would be a foolish and presumptuous
man who presumed to investigate their nearest and special causes.
But I do not hesitate to affirm that one who should investigate
the intermediate causes would discover much that might not
only give pleasure but also be very useful. ^ 'This then is the aim
that I have had in view in my investigation.

No one need be surprised that the subject of contagions was
neglected by our ancestors, for that is what happens in all
branches of learning. '"Those who lay the foundations can never
completely finish their work, since they are occupied in the pur—
suit of general facts and the first principles of things. And so,

: For his use of this word, which he probably introduced to medi—
cine, see Book I, note 4.

D