Chapter 1: Mental causation, multiple realization and physicalism
1.4 Kinds of multiple realization
1.4.2 Robust multiple realization
Fortunately for those who aim to argue for the autonomy of the special sciences, there is the possibility of what Francescotti (2014) calls robust multiple realizability. According to his definition, which I will call RMR, it obtains when there are no exclusive commonalities among the realizers of a higher-level property. Robust multiple realization obtains for higher-level property F and the class of realizer properties C iff:
„there are members, G and H, of C such that G ≠ H and it is nomically possible that: an instance x of G instantiates F and an instance y of H instantiates F, and there is no property J of C that x and y both instantiate and that is common only to instances of F” (Francescotti 2014:9, notation changed to match mine)
23 For such systems constant external energy flow is vital, without that they cannot maintain their integrity. They
are energetically coupled with their environment, so the core realizer together with other working components of the organism would still be insufficient for the realized property without certain features of the environment.
There is an insight from the discussion of mild multiple realization one should utilize here to amend this definition. Even though it is highly unlikely, it is possible that all realizers of F have something in common, but that commonality is irrelevant for the realization of F. In light of this possibility I would amend the definition as follows. RMR1:
F is robustly multiply realized by the class of realizer properties C iff there are members, G and H, of C such that G ≠ H and it is nomologically necessary that: if an instance x of G instantiates F and an instance y of H instantiates F, and there is a property J of C that both x and y instantiate and that is common only to instances of F the mutual manipulability criteria (MMi & MMii) fails to be true for the J and F
Robust multiple realization unquestionably justifies serious ontological antireductionist conclusions, but it is controversial that there are good examples of it. According to one influential camp of authors (Polger & Shapiro 2016), robust (or interesting) multiple realization is hard to find, while others (Aizawa & Gillett 2009) think that choosing the right framework it is easier to find examples. I will get back to this disagreement in the next section. Note, that if a property is robustly multiply realized, then there is no unified way to predict and explain the higher-level property in question based on lower-level information. This also means that it is impossible for the different realizers of the same higher-level property to be in the same kind of physical state. If the different realizers shared the same kind of lower-level state one could find a commonality among them that would allow for the use of a biconditional bridge-law.
There is at least one straightforward example in the literature that seems to fit the bill: the case of temperature. Nagel (1961) used the case of temperature as his paradigm
example for proper reduction, but later it became a much-debated case (see: Enç 1983; Bickle 1998). Let us take a closer look at this example. According to classical thermodynamics, temperature is only assigned to objects or volumes of gases in thermodynamic equilibrium. The temperature is identified to, is a measure of the motion energy of particles, more precisely the mean kinetic energy of the particles that comprise the volume of gas or object in question. However, the kinds of motion relevant for temperature are different for different kinds of matter, and the calculation of temperature from lower-level information requires different mathematical equations for different kinds of matter. This is the reason why philosophers in the 1980s started to cite temperature as an example of a multiply realized property. Bickle (2019: section 2.2) summarizes the example in a short passage, and unfortunately his reference authors like Enç (1983) do the same. It is worth providing a description that is a bit more detailed.
Temperature in an ideal gas is the mean molecular translational kinetic energy. This theory is well-known. But there is a difference even between how the temperature of different kinds of gases can be properly accounted for, as the above definition is only applicable to monoatomic, more precisely to noble gases. In gases that consist of diatomic molecules or of more complex molecules, rotational movement and vibrations of the compound structures also have to be accounted for. These molecules have more degrees of freedom, so they have more ways to absorb energy.
At a first approximation, temperature in a solid is the mean maximal molecular kinetic energy of the constituents. The molecules of a solid are bound in a lattice structure and restricted to vibratory motions. In actual fact the proper physical description utilizes so-called phonons (see: Fai & Wysin 2012:389-390). A phonon is a collective excitation in a periodic arrangement of particles in condensed matter, but the phonon-based description requires a
quantum mechanical treatment, quantization of the vibrations. So, temperature in solids is something entirely different from temperature in a noble gas.
Just to take a glance at further alternatives, radiation has temperature as it can be in thermodynamic equilibrium with a solid or a gas, and it is also something entirely different compared to solids or gases. There are no atomic or molecular constituents in radiation, its components are energetically coupled electromagnetic waves and for that reason that phase is dominated by electromagnetic interactions (see: Sklar 1993:351-353, 2015: section 6).
It is instructive to think through the example of temperature using the basic distinction between mild and robust multiple realization. Mild multiple realization requires less than what was discussed in connection to temperature above. Many would think that temperature in a gas is multiply realized by all the different micro-configurations that realize the same temperature value. This implies extremely small differences between realizers that are taken to be distinct realizers. According to those who think this way, even the smallest compositional difference between realizer systems is enough to reference them as distinct realizers. But it is also true that there is something invariant in all those different samples of gas, this is the reason why scientists could produce a bridge-law connecting temperature to an overall property of the constituents of a gas sample. The commonality is expressed by the mean (translational, etc.) kinetic energy of the constituent atoms. From this point of view, temperature in a volume of gas, temperature restricted to volumes of gas, is only mildly multiply realized. However, temperature as such in an unrestricted sense is robustly multiply realized, as it is captured by different basic models, explanatory mechanism in different kinds, phases of matter. The monoatomic gas with freely moving particles has different kinds of mechanisms to absorb energy compared to a solid with its phonon vibrations.
A more entrenched example of robust multiple realization is the case of human brains and computer brains, which one finds in almost all textbooks. Almost everybody agrees that if intelligent computers could be made, they would constitute an essentially different kind of realizer for intelligence. Only one condition needs to be satisfied for this to be the case. If the basic procedures by which a human brain and a computer carries out its operations are seriously different, then the view is indeed plausible.