Page 57 - Deep Learning

40 Introduction

that contains the germ. The mosquito flies to another person and bites again,
at which point some of the germs are inserted in that person’s body and begin
to multiply there; and so on. This narrative makes understandable several oth-
erwise inexplicable aspects of yellow fever, such as the timing and geographical
location of epidemics and the pattern of diffusion within each epidemic.
Consider next the explanation for the electrolysis of water: Pass electric-
39
ity through water and the water turns into hydrogen and oxygen gases. How
does this chemical transformation happen? As every chemistry student knows,
water molecules consist of two hydrogen atoms and one oxygen atom con-
nected via co-valent bonds, H O in the standard chemical formula. The elec-
2
trical current dissolves the bonds, causing the hydrogen and oxygen atoms to
drift apart. When two hydrogen atoms bump into each other, they bind, form-
ing one molecule of hydrogen (H ). Likewise, two oxygen atoms bind to form
2
one molecule of oxygen (O ). So two water molecules turn into two hydrogen
2
molecules and one molecule of oxygen.
These two explanations differ in content but share certain structural fea-
tures. Both explain by breaking down the observed change – the spread of
the disease, the transition from water to gas – into a succession of changes of
smaller scope or duration. I refer to the latter as unit changes. A unit change is
brought about by a process that we are willing to accept as given for the pur-
pose of explanation. In the medical example, the unit changes are the repro-
duction of germs inside the body and the abilities of mosquitoes to penetrate
human skin and to fly from person to person. In the chemistry example, the
unit changes are to dissolve and to form bonds between atoms. In this case,
the effects of the basic processes accumulate over large numbers of molecules
to produce the visible change that we call electrolysis of water. Briefly put,
such componential explanations break down a change of large scope into a suc-
cession of changes of smaller scope, each brought about by a different pro-
cess. The unit changes are components of the greater change to be explained,
and they produce that change as a cumulative result of their combined and
repeated action. Although the unit changes are taken as given for the pur-
pose of explaining some particular observed change, they are not atomic in
any absolute sense. Each can in turn be subject to analytic breakdown into
processes of yet smaller scope or duration (How do atoms share electrons in a
co-valent bond? How does the proboscis of a mosquito penetrate human skin?).
The second key feature of the two example explanations is that they spec-
ify the triggering conditions under which the unit changes occur. In the disease
example, the triggering conditions include the conditions that enable mosqui-
toes to breed, especially the availability of freshwater ponds. Another set of