Last time, we ended by discussing a certain type of argument for convergent realism known as retention arguments. These arguments rely on the following two theses.

Thesis 1 (Approximate Truth). If a certain claim appears an initial member of a succession of increasingly successful scientific theories, and either that claim or a claim of which the original claim is a special case appears in subsequent members of that succession, it is reasonable to infer that the original claim was approximately true, and that the claims that replace it as the succession progresses are increasing approximations to the truth.

Thesis 2 (Reference). If a certain term putatively referring to certain type of entity occurs in a succession of increasingly successful scientific theories, and there is an increasingly large number of properties that are stably attributed to that type of entity as the succession progresses, then it is reasonable to infer that the term refers to something real that possesses those properties.

As I noted last time, there are many ways a theory can retain claims (or terms) as it develops and becomes more successful. (We will allow those claims and terms that remain stable across radical change in theories, such as "paradigm shifts" of the sort described by Kuhn.) For example, the old theory could be a "limiting case" of the new, in the formal sense of being derivable from it (perhaps only with auxiliary empirical assumptions that according to the new theory are false); or, the new theory may reproduce those empirical consequences of the old theory that are known to be true (and maybe also explain why things would behave just as if the old there were true in the domain that was known at the time); finally, the new theory may preserve some explanatory features of the old theory. Convergent realists argue from retention of some structure across theoretical change that leads to greater success that whatever is retained must be either approximately true (in the case of theoretical claims such as Newton's Laws of Motion, which are "limiting cases" of laws that appear in the new, more successful theory) or must refer to something real (in the case of theoretical terms such as "electron," which have occurred in an increasingly successful succession of theories as described by Thesis 2).

Warning: Now I should note that I am presenting the convergent realist's position, and retention arguments in general, somewhat differently than Laudan does in "A confutation of convergent realism." In that article, Laudan examines the "retentionist" thesis that new theories should retain the central explanatory apparatus of their predecessors, or that the central laws of the old theory should provably be special cases of the central laws of the new theory. This is a prescriptive account of how science should proceed. According to Laudan, convergent realists also hold that scientists follow this strategy, and that the fact that scientists are able to do so proves that the successive theories as a whole are increasing approximations to the truth. He objects, rightly, that while there are certain cases where retention like this occurs (e.g., Newton-Einstein), there are many cases in which retention of this sort does not occur (Lamarck-Darwin; catastrophist-uniformitarian geology; corpuscular-wave theory of light; also, any of the examples that involve an ontological "loss" of the sort emphasized by Kuhn, e.g., phlogiston, ether, caloric). Moreover, Laudan points out that when retention occurs (as in the transition from Newtonian to relativistic physics), it only occurs with regard to a few select elements of the older theory. The lesson he draws from this is that the "retentionist" strategy is generally not followed by scientists, so the premise in the retentionist argument that says that scientists successfully follow this strategy is simply false. I take it that Laudan is correct in his argument, and refer you to his article for his refutation of that type of "global" retentionist strategy. What I'm doing here is slightly different, and more akin to the retentionist argument given by Ernan McMullin in his article "A Case for Scientific Realism." A retentionist argument that uses Theses 1 and 2 above and the fact that scientific theories are increasingly successful to argue for a realist position is not committed to the claim that everything in the old theory must be preserved in the new (perhaps only as a special case); it is enough that some things are preserved. (McMullin, in particular, uses a variation on Thesis 2, where the properties in question are structural properties, to argue for scientific realism. See the section of his article entitled "The Convergences of Structural Explanation.") That is because the convergent realist whom I am considering claims only that it is reasonable to infer the reality (or approximate truth) of those things that are retained (in one of the senses described above) across theoretical change. Thus, it is not an objection to the more reasonable convergent realist position that I'm examining here (of which McMullin's convergent realism is an example) to claim that not everything is retained when the new theory replaces the old, and that losses in overall ontology occur with theoretical change along with gains.

That said, there are still grounds for challenging the more sensible, selective retentionist arguments that are based on Theses 1 and 2. I will concentrate on the issue of successful reference (Thesis 2); similar arguments can be given for the case of approximate truth (Thesis 1). (Follow each objection and reply with discussion.)

Objection: The fact that a theoretical term has occurred in a succession of increasingly successful scientific theories as described by Thesis 2 does not guarantee that it will continue to appear in all future theories. After all, there have been many theoretical terms that appeared in increasingly successful research programs (phlogiston, caloric, ether) in the way described by Thesis 2, but those research programs subsequently degenerated and went the way of the dinosaurs. What the convergent realist needs to show to fully defend his view is that there is reason to think that those terms and concepts that have been retained in recent theories (electron, quark, DNA, genes, fitness) will continue to be retained in all future scientific theories. However, there is no reason to think that: indeed, if we examine the history of science we should infer that any term or concept that appears in our theories today is likely to be replaced at some time in the future.

Reply 1: The fact that many terms have been stably retained as described by Thesis 2 in recent theories is the best reason one could have to believe that they will continue to be retained in all future theories. Of course, there are no guarantees that this will be the case: quarks may eventually go the way of phlogiston and caloric. Nevertheless, it is reasonable to believe that those terms will be retained, and, what is more important, it becomes more reasonable to believe this the longer such terms are retained and the more there is a steady accumulation of properties that are stably attributed to the type of entities those terms putatively designate. Anti-realists such as Laudan are right when they point out that one cannot infer that the unobservable entities postulated by a scientific theory are real based solely on the empirical success of that theory, but that is not what scientists do. The inference to the reality of the entities is also a function of the degree of stability in the properties attributed to those entities, the steadiness of growth in that class of properties over time, and how fruitful the postulation of entities of that sort has been in generating accumulation of this sort. (See McMullin, "The Convergences of Structural Explanation" and "Fertility and Metaphor," for a similar point.)

Reply 2: It's not very convincing to point to past cases in the history of science where theoretical entities, such as caloric and phlogiston, were postulated by scientists but later rejected as unreal. Science is done much more rigorously, rationally, and successfully nowadays than it was in the past, so we can have more confidence that the terms that occur stably in recent theories (e.g., electron, molecule, gene, and DNA) refer to something real, and that they will continue to play a role in future scientific theories. Moreover, though our conception of these entities has changed over time, there is a steady (and often rapid) accumulation of properties attributed to entities postulated by modern scientific theories, much more so than in the past. This shows that the terms that persist across theoretical change in modern science should carry more weight than those terms that persisted across theoretical change in earlier periods.

Reply 3: The objection overemphasizes the discontinuity and losses that occur in the history of science. Laudan, like Kuhn, wants to argue there are losses as well as gains in the ontology and explanatory apparatus of science. However, in doing so he overemphasizes the importance of those losses for how we should view the progression of science. The fact that scientific progress is not strictly cumulative does not mean that there is not a steady accumulation of entities in scientific ontology and knowledge about the underlying structural properties that those entities possess. Indeed, one can counter Laudan's lists of entities that have been dropped from the ontology of science with equally long lists of entities that were introduced and were retained across theoretical change. Our conception of these entities has changed, but rather than focusing on the conceptual losses, one should focus on the remarkable fact that there is a steadily growing class of structural properties that we attribute to the entities that have been retained, even across radical theoretical change. (See McMullin, "Sources of Antirealism: History of Science," for a similar point.)