Book I CONTAGION 27

then surely they resort to the unnecessary and assign an incon—
gruous cause. For if contagion has been correctly defined, it
follows that what is developed in the second thing must be of
the same sort as was developed in the first thing, and that the
principle in the two things is the same, and it must be the same
in the fourth and fifth and so on, in everything, in fact, that
receives the contagion. But nothing spiritual can effect this
per se, though by means of something accidental (non—essentia!)
the spiritual may certainly destroy and even dissolve some com—
bination by putting to flight certain contrary principles; for in—
stance a stench can do this, or the mere sight of the animal
called catablepha, as has been said in my work 'On Sympa—
thies'. " But the spiritual cannot generate in a second body the
same sort of thing as was in the first body, for all generation
occurs through primary qualities.

Now, in these contagions, not only putrefaction must be
produced, but from the original germs other germs must be be—
gotten and propagated that are similar to those former germs
both in their nature and combination; just as in an animal
organism the spirits'* generate in its blood others like them—
selves, which no spiritual quality per se can effect. For which
reason we must suppose that, in contagions that infect at a
distance, there must be one and the same common principle
and the same mode of infection, and by means of known quali—
ties; and we see that Aristotle and Galen without doubt thought
so too. Vet the germs of these contagions do differ from those
that infect by contaet only, and also from those that infect by
fomes only. But first let us state in what way they can be
carried to a distant object, and how they seem to move in &
circle; how it is that they are endowed with such force and
penetrate so quickly; of what sort are their analogies; and last—
ly we shall define the difference between them and other germs.