among philosophers of biology and biologists. Accounts of these three
dimensions have generally been based either on intuition or on evolutionary
theory (Hull 1992; Godfrey-Smith 2009), leaving aside, in most cases, other
biologicalfields, including physiological ones. In all these accounts the idea
of“cohesion”seems particularly diverse and equivocal: a range of concepts
have been suggested to elucidate this idea, including“functional integra-
tion” (the fact that parts are interconnected and interdependent) (Sober
1991 ),“near-complete decomposability”(the fact that interactions between
parts of the individual are stronger than interactions between parts of the
individual and the environment) (Simon 1969; Wimsatt 1972), and“coop-
eration and absence of conflict”(Queller and Strassmann 2009), but these
concepts only partly overlap, and they are not always easy to define and to
apply.
There is nowadays a quasi-consensus that biological individuality is
question-dependent and practice-dependent, can be realized at different levels,
comes in degrees, and should not be based on intuitions or on an anthropocentric
approach (Pradeu 2016a; Lidgard and Nyhart 2017). I agree with this consen-
sus. The specific question raised here is in what sense and to what extent the
domain of immunology contributes to addressing the problem of biological
individuality, essentially from a physiological point of view. (Section 5dis-
cusses how to combine this approach with other approaches to biological
individuality, including evolutionary ones.)
The main claim of this section is that immunology makes an important
contribution to the definition of biological individuality insofar as it sheds
light on all three dimensions discussed above, namely countability, delineation,
and cohesion. The three main activities of the immune system participating in
the individuation of biological entities can be called“filtering over entry,”
“filtering over presence,”and“promotion of cooperation”(by“filter”I mean
allowing or, on the contrary, restraining the entry or presence of something, as
explained below). AsFigure 3.1indicates, these activities map onto countabil-
ity, delineation, and cohesion.
“Filtering over entry”refers to the fact that the immune system constantly
patrols the interfaces of the organism with the environment (skin, gut, and so on)
and determines which exogenous elements can enter the organism and which
can’t. It thus plays a decisive role in the delineation of the organism’s bound-
aries and the possibility of counting it as one single entity.
But the immune system does much more than control entries into the organ-
ism at interfaces. In a process that can be called“filtering over presence,”the
immune system constantly monitors the motifs expressed by the cells present in
all tissues and body compartments as well as their intracellular content and
Philosophy of Immunology 21