Introduction

I have argued in my (1999) that scientific realism consists of three theses.

The Metaphysical Thesis: The world has a definite and mind-independent structure.

The Semantic Thesis: Scientific theories are truth-conditioned descriptions of their intended domain.

S. Psillos (s)

Department of Philosophy and History of Science, University of Athens, Athens, Greece e-mail: psillos@phs.uoa.gr

© Springer International Publishing AG 2017

E. Agazzi (ed.), Varieties of Scientific Realism,

DOI 10.1007/978-3-319-51608-0_11 theoretical terms featuring in theories have putative factual reference. So if scientific theories are true, the unobservable entities they posit populate the world.

The Epistemic Thesis: Mature and predictively successful scientific theories are well-confirmed and approximately true. So entities posited by them, or, at any rate entities very similar to those posited, inhabit the world.

Briefly put, the rationale for these three theses is the following. Scientific realism is a tri-partite philosophical view. It is a view about the world; a view about scientific theories; and a view about the cognitive achievements of science. Being a view about the world, scientific realism is a species of realism; hence, it must imply some commitment to a realist understanding of the world and the relation of science to it. More specifically, it must honour the realist commitment to the mind-independence of the world. Being a view about scientific theories, scientific realism should be committed to realist semantics for scientific theories, viz., a commitment to an irreducible, truth-conditional account of the meaning of theo­retical terms. Being a view about the cognitive achievements of science, scientific realism should be committed to some kind of epistemic optimism, viz., the claim that science has been on the right track; it has succeeded in offering us knowledge of the world and in particular knowledge of the parts of the world which are not detectable by the naked senses.

Though conceptually distinct, these three theses form a tight network. For instance, what's distinctive about scientific realism is the view that the world of appearances (the world as it is revealed to us through our senses) does not exhaust the content of the world—this content being, by and large, unobservable, though no less real. Moreover, this world is by and large independent of us and the various ways scientists use to describe it and it is this independent world which renders theories true, insofar as they are true. Taking science as a cognitive endeavour which succeeds in describing an independent world is not a priori true—nor his­torically constitutive of science. It is a philosophical stance towards science which has its basis on the fact that scientists developed theories which posited unob­servable entities to explain the behaviour of observables; but it is also grounded on a certain semantic attitude towards theories—viz., taking them in an irreducible (face-value) way. This implies that theories—qua attempts to describe the inde­pendent world—have excess content over and above whatever can be expressed in an observational language, insofar as we can make sense of a theory-free obser­vational language.

In my own work over the years I have mostly dealt with the epistemic thesis and to a lesser extent with the semantic thesis. Insofar as I have dealt with the meta­physical thesis, I have taken it to be a minimalist one. It amounts to a declaration of independence: the world is mind-independent. In this paper, I want to say a bit more about the metaphysical component of scientific realism and in particular to develop and defend the view that the realist claim of mind-independence is captured by what I have called ‘the possibility of divergence', viz., the possibility of a gap between what there is in the world and what is issued (or licensed) as existing by a suitable (even ideal) set of epistemic practices and conditions.

To fix our ideas, I will break up the realist commitment to mind-independence into two components: irreducible existence and objective existence.

Irreducible existence means existence in its own right; that is, not dependent on the existence of something else. To say that a kind of entities X exist irreducibly is to say their existence is not grounded, in whatever way, in the existence of another kind of entities Y; or it does not depend on the existence of another kind of entities Y. Hence, to say that entity K exists reducibly is not to say that K is not real but that its reality depends on (or is reducible to) the reality of some other entity (or entities). As we will see in more detail later, for traditional idealism material objects are not irreducibly real, while for traditional materialism, they are.

Objective existence means existence independently of epistemic or cognitive conditions that require the verification, recognition or knowledge of existence. To say that a kind of entities X exist objectively is to say that their existence is not in any way constitutively connected with epistemic procedures Ô that allow or enable cognizers to decide, or otherwise certify, their existence. As we shall see in more detail later on, verificationist anti-realism denies the condition of objective exis­tence, without thereby denying the condition of irreducible existence. (It is also consistent to deny irreducible existence without denying objective existence—an example would be the mind-body identity thesis.)

In light of Michael Dummett's quotation in the beginning of the paper, I want to contrast scientific realism with two opposing views: idealism and verificationism. I will be relatively brief with idealism and I will focus my attention on verifica- tionism. My chief aim will be to show that a verificationist version of scientific realism, though honouring the condition of irreducible existence, compromises the condition of objective existence and hence it is in conflict with the realist demand for mind-independence.

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