William Seager

LEIBNIZ AND SCIENTIFIC REALISM

I want to enlist Leibniz's aid In answering a vexing question in the philosophy of science. It must be said at the outset that no one could conf1dently expect Leibnlz to speak directly to any particular philosophical

problem In this stili young discipline. We will do better to search Lcibniz

for hints and sugg£'stions -- and these we shall f1nd aplenty,

A second prdimlnary point. may be In order. I will discuss Lcibnlz's

views on natural science, and especially his views on the truth of scientific theories. Thus I want to mis£' and dispose of the objection that Leibniz really thought that all sc\£'ntif1c UJ{'ories were false for the simple reason

that they all dealt with the m{'re world of appearance or phenomena.

These app£'aranc{'s all depend on the states of the monads, or are stat{'s of the monads (this is, of course, a complicated story), and only the monads actually exist. I suppose that this objection Is literally correct, but

stultifying. The distinction betw{'en appearance and reality Is also drawn, by Leibniz, at the level of the monadic perc{'ptions (sec, for example, -On

the Method of Distinguishing Heal from Imaginary Phenomena-.)I This is the level where science Is at horne -- and it Is a level where the conc{'pts of truth, falsity and evidence all apply.

Th£' qu('stlon at Issue Is simply whether we ought to bdicve sciPntif1c

theories to be true or merely £'rnpirically ~ukquate. Of course, such a bald and unqu?,lif1ed pres£'ntation of th{' question leaves it unansw{'rabl{'. Presumably no one would hold that any extant scienUf1c UH'ory Is cithpr true or empIrIcally adequate for the obvious reason that no extant thpory enjoys unmitigated empirieal success. But it's no false dilemma we have here -- only we need to make our qllesUon more prpc\se.

Following van Fraa.'lsen let us say that a tiJ{'ory is empirically adequate

315

K. Okruhlik and J. R. Brown (eds.) , The Natural Philosophy of Leibniz. 315-331. © 1985 by D. Reidel Publishing Company.

:316 William Seager

if all observable phenomena fit inside some model of the theory, or if

"what the theory says about the observable things and events in this

world is true".2 (This last is a Quotation from van Fraassen's book, The Scientific Image, a recent and powerful attack on scientific realism.) Note that empirical adequacy extends across all time and space, If we

drop the "observable" from the above, we arrive at what It Is for a theory

to be true simpliciter: not suprlslngly, a theory Is true simply If what it

says about the world Is true.

We still fac(~ the complicating difficulty that no extant theory is really

deserving of our allegiance either with regard to truth or to empirical adequacy. But we may adopt van Fraassen's characterization of realism,

and, as he calls his own position, constructive empiricism, to further

clarify our question, which becomes: Is it the aim of science to discover

true theories or merely empirically adequate ones?

There would be no problem If theories restricted themselves to

statements about the observable aspects of the world -- but of course they

do not, and Instead promiscuously entertain such dubious entities as

atoms, humours, phlogiston, electrons, microbes, and on and on. Our

problem thus becomes the Question of whether sclence's employment of

theories which mention unobservables amounts to the postulation of the existence of these things, or is merely a useful way to develop greater

empirical adequacy.

The scientific realist is one who holds that when a scientific theory

which mentions an unobservable Is advanced, this advancing Includes the

positing of the existence of the unobservable.

I think there can be no doubt that Leibnlz subscribed to scientific

realism so construed. I believe this not because he ever actually says that, In general, the unobservables of scienttnc theory are to be taken as posits

(though he may somewhere -- the Leibnlz concordance might help here),

but rather because of his free use of them In the construction of proto­explanations -- explanations which do not have the backing of any serious or well developed scienttnc theory. I1is" A New System of the Nature and

the Communication of Substances, as well as the Union Between the Soul and the Body", for example, Is one place, of many, where Lcibnlz puts

forth his doctrine that at the death of an animal the "organic machine" is preserved as a home, as it were, for the soul. This, of COllrse, is not

observed by us -- but the explanation given is that the organic machine of

the animal shrinks at death to "a size so small It escapes our senses just as

Leibniz and Scientific Realism 317

It did before birth W (L p. 453). Perhaps In a more sclentlnc vein we have Lclbnlz saying in wRefiections on the Common Concept of Justlce w:

This Is also true when one looks at the brain, which must undoubtedly be one of the greatest wonders of nature, since It contains the most immediate organs of sense. Yet one nnds there only a confused mass In which nothing unusual appears but which nevertheless conceals some kind of nIaments of a fineness much greater than that of a splder's web which are thought to be the vessels for that very subtle fluid called the animal spirits. Thus this mass of brain contains a very great multitude of passages -- and of passages too small for us to overcome the labyrinth with our eyes, whatever microscope we may use. For the subtlety of the spirits contained in these passages is equal to that of light rays themselves. Yet our eyes and our sense of touch show us nothing extraordinary In the appearnace of the brain. (L p. 565)

(Note here how Lelbnlz gives an example of unobservablllty which is based on theory -- a theory of light and vision.) Or again, In WOn the Elements of Natural Sclencew Leibnlz proposes that the explanation of the general operation of the human body, which Is, after all, nothing but an Whydraullc-pneumatlc machlne W Is In terms of certain Whidden ways ... In which composite things are dissolved Into Insensible partsW (L p. 283).

Lelbnlz Is completely unselfconsclous In his readiness to appeal to Imperceptible aspects of nature, a readiness he shared with his contemporaries. Descartes, for example, explains rarefaction by appeal to Insensible pores In bodies which can be opened to be filled by other matter, rather like a sponge (Principles, Pt. II, Principle 7) and Lelbnlz opined that Descartes had thus wadmirably explalned w rarefaction. In support of this mode of explanation Descartes says: wThere is no compelling reason to believe that all bodies that exist must affect our senses W (ibid.)

Th us we are at least free to postu late their existence when necessary. Lelbnlz, In his criticism of the Principles, passes by this remark without comment -- It Is too evident even to praise.

Here It my be worthwhile to forestall another objection stemming from Lelbnlz's overall metaphysics. Did he not hold that everything is perceived or represented In the perceptions of the monads (indeed, of any monad), and hence that there can be nothing which does not affect our

William Seager

sens('s'? The reply, which I draw from Parkinson's "The Intellectualization

of Appearances" ,3 involves the distinction between distinct and confused

perceptions. Everything is perceived only in the sense of confused

perception, which is not sensation. When a perception becomes distinct it

becoflH's a sensation (Leibniz says: "If a perception is more distinct it

makes a sensation" ).1 Since sensations are necessary for observation, the

noUon of an unobservable is perfectly acceptabie to Leibniz.

The belief in unobservables which can somehow account for the

phenoITIena we actually observe is not of course peculiar to the

sevenj('('nth century. It is found everywhere. A fragment from

A naxagoras is nice; it reads: "appearances are a glimpse of the unseen".

The bare proposition that there are unobservablcs which have something

to do with appearances is found everywhere because it is so obviously

true. Observe the dust motes in a shaft of sunlight from across a room.

As you walk closer more and more appear to you. It is, to me at least,

overwhelmingly plausible that these motes grade off in size until some are

t,ruly unobservable. I hope to show that the bare plausibility of the

existence of unobservables is one of the b1l<'iic underpinnings of scientific

realism.

And this plausibility ITIight be thought enough of itself to answer our

original question, for given the legitimacy of postulating unobservables,

and the claim that any theory which is really empirically adequate is very

likely to be true the dilemma of our original question appears to

disappear.

Yes -- but suppose that for any theory 'I' there was a second theory T'

inconsistent with T but which utterly agreed with T about all observable

things and events. That is, what if, for any theory there was an

empirically equivalent rival? This is a possibility, and so long 1I<'i the test

of scientific theorising is empirical success there appears to be no ground

for choosing one theory over the other. I will take a moment to illustrate this with a concrete example used by

van Fra1l<'isen, one striking in its simplicity. Consider Newtonian

mechanics and gravitational theory. Within this theory uniform motion

within absolute space Is undetectable. This observation generates a

multitude of empirically equivalent theories, each one positing a different

uniform velocity In absolute space for, say, the sun. That cach such

theory is empirically equivalent is almost obvious (see van Fra1l<'isen's

discussion in chapter 3 of The Scientific Image) and each is clearly

Leibniz and Scientific Realism 319

inconsistent with any other. If science alms at truth then selence has

stalled. This coneluslon Is escapable only if certain non-empirical determinants

of theory choice can be brought Into play here. But again. If the aim of

science is truth these non-empirical criteria will have to be evidential, that

is, h:we a bearing on the probable truth or falsity of theories. After all,

there Is no question but that we shall choose that theory which Is easiest

to usc from any empirically equivalent grouping, yet eik'ie of usc is not

obviously linked to truth. Let us call a non-empirical but evidential

criterion of theory choice a Wmetaphyslcal principle.

Lelbniz's reply to van Fraassen's example illustrates the use of

metaphysical prlnelples to evaluate theories. In the correspondence with

Clark Lelbniz argues that space Is not absolute on the grounds that, if it

were, God could wcause the whole universe to move forward in a right line

or any other line, without making otherwise any alteration in it W (L p.

688). This would be, however, to make a wchimerical supposition w. Of

course, It follows that the assumption that the sun has any particular

velocity In absolute space would be equally chimerical. Leibniz advances

two related arguments in support of this; one from the principle of the

identity of Indlscernlbles, another from the principle of sufl1cient reik'iOn.

As to the latter, God could have no reason to favour the sun with any

particular velocity (including velocity zero) over any other, and hence

could not. And further, since Newton's theory leads Inescapably to a

world in which God faces this Impossible choice It is Newton's theory, and

hence everyone of the empirically equivalent theories considered above,

which must be wrong.

It Is tempting to generalize Lelbnlz's argument to all cases of empirical

equivalence. Suppose T and T' are empirically equivalent theories. Then

the state wherein T holds Is Indiscernible from that wherein T' holds,

hence there Is no sufficient reason for T holding rather than T', or vice

versa. By the above reik'ioning, both T and T' must be false. This

attempt falls to convince, of course, but for a reason which supports our main point. The crucial claim that the states wherein T and T' hold are indiscernible Is equivocal. They are so only empirically. The argument

will fail where there is a non-empirical discernibility which is determinative for a rational and moral creature. For It is then that God could, and would, choose between making T or T' true of the world. (Creating a world Is precisely to make a theory true. In van Fraassen's termInology, God is a model builder.) From the human standpoint, a

320 William Seager

non-empirical but determinative dlscerni bility is a criterion of theory choice which is non-empirical but evidential -- that Is, a metaphysical principle.

For Leibnlz the metaphysical prlneiples, such as that of continuity, operate as a sort of a filter through which theories or hypotheses must pass. But this filter Is not an accurate gauge of empirical adequacy, for

Leibnlz admits that a false hypothesis may nonetheless be useful at the empirical level (L p. 283-81). Ilowever, the filter docs play a clearly

evidential role, for with regard to the Cartesian laws of Impn,ct Leinbiz

shows how the prineiple of continuity cn,n be used so that "the truth In these matters can be outlined In advance, as It were, or as a kind of

prelude ..... (L. p. 102}."

No one Is surprised to sec Leibniz employing metaphysical principles in theory evaluation. For everyone knows the rationalists -- sitting In their armchairs, strenuously applying pure reason to the world and faultlessly

deriving absurdity after absurdity. Hut our neutral definition reven,ls that the use of metaphysical principles Is no less, or not much less anyway, common today. I herc offer two examples which have figured prominently In the discussion of this question.

l<:xample 1. Wide-spread In discussions of scientific realism Is the so­called principle of the common cause which asserts that science Is charged with explaining any positive correlation (or perhaps, any correlation at all) between objects or events by way of a common cause of both. So we explain the correlation between the readings of two Independent

thermometers by the obvious common cause. This Is a natural pattern of explanation In everyday life about oberservable phenomena, but if applied in general will demand the postulation of non-observables. For without this postulation there Is no explanation. The postulation of unobservables by a given theory will in some cases, then, determine theory choice among

empirical equivalents. It is clearly a non-empirical determinn,nt, yet it is also evidential, for to postulat,e an unobservable thing Is to postulate the truth of statements asserting the existence of the unobservable; the theory which docs not posit them will thus miss the truth. The principle of the common cause so applied Is thus a metaphysical principle. In fact, I cannot pass It by without mentioning the obvious links between this principle and the principle of sufficient reason.

Example 2. Consider now the argument vigorously advanced by

R. Boyd6 in support of scientific realism. Science has been and continues

Leibniz and Scientific Realism 321

to be remarkably successful at the empirical level. This fact calls for an

explanation -- the best being simply that science Is true, or, as Boyd more

coyly puts It, approximately true. If science at a given time is

approximately true then new theories and extensions of old theories which

are win the tradition W or relevantly similar to older theories are to that

extent more plausible, more likely to be true than exotic rivals. Once again, this principle of tradition-bound plausibility provides a non­

empirical but evidential criterion of theory selection -- it is what we call a

metaphysical principle.

There are many other examples to be found in discussions of scientific realism, but these two are particularly important, not least because of

their wide range of applicability. Although the existence of such

principles is not in doubt, their status is. The anti-realist can take issue

with them in various ways, always attacking their evidentiality. Van

Frassen argues in The Scientific Image that, since current science rejects

a full application of the principle (it leads to the dreaded Whidden

variables W in Quantum Mechanics), the principle of the common cause is

only a me'thodological maxim for theory production; the principle' is

reduced wfrom a regulative principle for all scientific activity to one of its

tactical maximsw.i Boyd's principle is undercut at the first step -- the

impressive empirical success which science currently enjoys is not necessarily best explained by the wapproximate truth W (a rather murky

notion anyway) of theories. Here, van Fraassen suggests to us instead

that a Darwinian explanation of scienUfic success may be just as plausible.

In fact, the anti-realist can point to a more or less continual

undercutting of one metaphysical principle after another throughout the

history of science. Leibniz's principles suffered this fate rather early on,

and to be thoroughly scientific we ought to reject ali such extra-empirical

determinants of theories and with it reject the outmoded notion that

science alms at anything more than empirical success. Van Fraassen charges that: Wreallst yearnings were born among the mistaken ideals of

traditional metaphysics w.8

The battle lines can be clearly drawn. First, the simple postulation of unobservables which in some way or other account for observable phenomena is acceptable to reason.

truth that some such entities exist. More, it appears to be an obvious

(The doctrine of "sclentinc entity realism w as it is sometimes called may stop here, but if so it is not a very

bold position.) If this were not so there would never have been a problem

322 William Seager

of scientific realism, for without such postulation emplrleal adequacy and truth never diverge. The anti-realist counters with the claim that for any theory there are empirically equivalent and incompatible rivals. This by itself does not show that the aim of science is not truth but rather empirical adequacy. But if one also supposes that the sole evidential criterion of theory choice Is empirical success this conclusion is reasonable. For if we have a vast number of scientists spending their time constructing theories which aceount for and predict observable things and events (and it does seem that they spend very little of their time searching out empirical equivalents to pre-existing theories -- that is apparently reserved for philosophers), and the sole measure of success Is just how well their theories do produce such accounts and predictions It is hard to resist the inference that their aim is the production of these accounts and predictions.

So we see why the realist attaches such importance to the metaphysical principles. I also believe that Lelbnlz was aware of this particular problem, as evidenced by his cautious endorsement of the so-called "conjectural method a priori" (see L. p. 283). This method is essentially the hypothetico-deductive method now much praised, but with regard to it Leibniz warns that "the same effect may have several causes". It is this very weakness that prompts Leibniz to relegate this method to "stop-gap" status -- useful hypotheses generated In this way are merely "to be presented, in the interim, in place of the true causes". The degree of probability these hypotheses can attain is apparently very great though -­Leibniz says they may be "morally certain" for example. He may be saying no more here than that such hypotheses cannot be demonstrated, which is, after all. true of all our scientinc knowledge until we arrive in the" better life".

In any event, to carry out his attack. the anti-realist must convince us that (1) for any thpory there are empirically PQuivalent rivals and (2) extra-empirical criteria of theory choice are non-evidential. Further, as I shall argue below, even if (I) and (2) can be supported UJ('Y do not necessarily lead to anti-realism, for (I) can be construed in a weakpr and a stronger way -- the anti-realist may require the strong sense for his anti­realist argument but the wPak spnse may be the only plausible form of (I).

The central issues that arise here obviously Involve the status of methods of comparison among competing theories. I want to invpstigate these issues from a standpoint which provides an abstract but intuitively apppaling picture of a fkld of comp('ting theories. A particularly striking

Leibniz and Scientific Realism 323

picture can be borrowed from computer science: it stems from a problem

solving technique, especially used in arUncial intelligence, called "I!ill­

climbing".

The technique is quite nicely illustrated by the task of tuning into a

television station. The me-asure of success is the quality of the picture; the

me-ans to achieve succe-ss are, let us say, adjustments of the tuning control

and the brightne-ss control. Abstractly speaking, we have a problem space

of three dimensions -- one dimension for each parameter mentioned above.

The plane formed by two adjustment axes will be dotted with hills

representing the available channels. The hili-climbing technique involve-s

moving through the problem space until one arrives at a "peak" -- a

position where the associated control adjustments produce the best

possi ble pietu re. Th is can be rough Iy illustrated so:

,. - -

--~~~------~---------------------J , ,

We can adopt this abstract pieture and provide an encompassing vkw of

competing the-ories. What is the mcasure of success of a theory? It is

emprical adequacy. Both realist and anti-realist can agree on this, for

while they disagree about the ultimate aim of science it is clear that the numbe-r one measure of success is the theory's ability to account for

observable phenomena.

Thus we shall have empirical adequacy as the "up" axis which

conventionally repre-scnts the ITlcasure of succe-ss. Needkss to say, UJ('rc is

no presumption I,hat this is the only criterion of scie-ntific success. I do

assume here, however, that Ul<'ories ('an be graded as to how empirically

ad('quate UJ('Y are. This s('elTlS rpasonable -- the notion of how near an

321 William Seager

empirical prediction is to the truth is clear enough for this purpose, and

can often bc quantined. (We hear scientists talk, for example, of a

measurement, being within two standard deviations of the prcdicted

value.)

The other axcs are much more difficult to label. A first point is that

there is no need to restrict them to two -- the hill-cHm bing concept can be

applied to a space of any dimension. The other axes can be simply

described as fcatures of theories which serve to differentiate them. Such

features are difficult to isolate. Simplicity might be one such feature,

whatever simplicity of theories is. In general, the metaphysical principles

themselves wllJ usually mark out a possible axis of comparison. The

principle of continuity, for example, Indicates an axis whereon theories are

differentiated by whether they permit "nature to move by leaps", as it were, or how much of this they permit, or in what aspect of theory they

permit It, and so on. However, as we are not trying 'to program a

computer to produce our theoric's for us, we can afford to be unspecific

about the nature of the various comparison axes. It Is not a very

contentious claim that theories can be compared one to another in a large

number of ways -- what is contentious is the status of these modes of

comparison, principally, whether they are evidential or not.

Having established this set of axes such theory t,akes its place as a

point in the problem space -- since the problem here is to achelve

empirical adequacy we might call it empirical adequacy space, or just

adequacy space. Theories which are near to each other vis-a-vis the

various comparison features but which differ In empirical adequacy will

form the hills of our imaginary landscape -- from the unimpressive mound

of impetus theories to the himalayan peaks of Newtonian mechanics,

quantum mechanics and relativity theory. Empirically equivalent theories

will lie on a plane cutting the axis representing empirical adequacy. The

steeper the hllJ around a theory the fewer competitive rivals of the same Ilk. An absolutely unique theory remains an Isolated point In adequacy space with no geography around it.

Certain theses drawn from the philosophy of science can find a place in this picture. Recall that the notion of hllJ-cllmbing Is the problem solving technIque associated with an abstract problem space. In computer

applications, the problem space Is rigorOUsly defined and the computer proceeds by taking what is called a standard step at each stage of the

search for a solution. After each step, the "altitude" is measured, and the direction of the next standard step Is varied in Just the way you would

Leibniz and Scientific Realism

expect. Now, we might usc this progression by cautious steps as a

metaphor for Kuhn's normal science. Hill-climbing fails in terrain like

that at A (on the diagram), for every standard step leads down, yet A is

only a local peak. Perhaps this Is the situation when the procedures of

normal science can no longer successfully attack the problems nature Ii,

posing for a given theory or range of theories. Seienfic revolutions are

then the mounting of an expedition up an altogether new peak -- though

the mechanisms by which this is accomplished are far from clear of course.

This picture can also be used to reinforce van F'raassen's rejection of

Boyd's view that when a seience reaches a ccrtain level of pmpirical

sllccess it Is then approximatply true. Since deeply incompatible theories cannot all be approximately true (on any reasonable construal of

"approximately") this amounts to a claim about the geography of

adequacy space: namely that there is a level of empirical adequacy above

which there is only one peak (the peak need not be too stepp; for

presumably similar, even if Incompatible, theories can all be

approximately true -- the various versions of general relativity might be

an example of this). Boyd's argument Is that only the approximate truth

of a seience can explain its empirical success, but this seems to beg the

question since It is cogent only If the geography Is just as he needs to

argue for; and no Independent rea.<;ons for mapping out adequacy space in

this way have been given.

But the diagram also suggests a problem about the anti-realist thesis

(1) -- that every theory has empirically equivalent rivals. Consider the

region B. This plateau represents a number of quite similar and

empirically equivalent theories, and can serve to represent the infinitely

many Newtonian theories, discussed above, which differ only in the

postUlated absolute velocity of the sun. It is possible that all empirically

equivalent theories of any degree of empirical success fall on such plateaus.

(This is so because of the gappiness which is possible in adequacy space -­

theories, or groups of theories can float in it.) If so, we have only a very

weak empirical equivalence thesis and, I think, one which will not support the anti-realist position. f;'or all such theories say essentially the same thing about the world. And they are just what the realist ought to expect given that there are elements of his theory which arc supposed to be unobservable. In such a case it would be acceptable for the realist to admit that one could never know, say, the sun's absolute velocity but that this limitation docs not Impugn the general Newtonian world picture.

Whichever theory Is selected it leaves us with essentially the same world.

William Seager

In this example the pieture of the world which is common to them all is

that theory in which the statement, "the velocity of the sun is v" is

replaced by the statement "there is a velocity, greater than or equal to

zero which the sun has". Now, whether or not empirically equivalent theories always range

themselves onto plateaus I can't say. Van Fraassen refers to Ellis's

version of Newtonian cosmology in which no gravitational force is postUlated but in which all motions are the same (the laws of motion incorporatc gravitation), and Ellis's theory does not really seem very

similar to Newton's. Of course, once a theory exists it may be easy to produce empirically

equivalent rivals to it. Is producing rivals in such a way some kind of

trickery? As a sort of limiting case van Fraassen provides a method for

producing any theory's empirically equivalent rival. Suppose we have a

theory T. Then we can produce the rival thus: let T' be the theory which

assrrts that T is empirically adequate. Where is this theory to be placed

in adequacy space?

Leaving the anti-realist thesis (1) for the moment let us consider the

action of the metaphysical principles within adequacy space. The

metaphysical principles restriet the range of acceptable theories, but

without regard to empirical adequacy. Thus, for at lea.'lt some of the

principles, they outline a subplane of the theory comparison plane which

is a sort of no-man's land. Theories In this region are not to be believed

(we may, of course, use them for prediction). [Examples Indicated on

diagram] Not every metaphysical principle operates In this way. For example,

the principle of sufficient rea.'lon Is supposed to rule out terrain like that

at B but the principle does not clearly mark out a subspace. But a related

principle perhaps does have an associated subspace. Some theories explain

more than others, and such explanatory power is a virtue (though, again, it may not be an evidential virtue). We might suppose that some theories

are to be rejected If they leave too much unexplained. Of course, It Is on some such principle that the van Fraassen empirical equivalent of a theory might be rejected -- for the theory T' which says only that T is empirically adequate leaves much unexplained that T presumably can

explain, though only in terms of unobservables. The modern metaphysical prlnclple which most clearly embodies this

demand for explanatory power Is that of the common cause. In The

Scientific Image, van Fraassen shows that current science does not hold

Leibniz and Scientific Realism :327

that this principle must have universal application. That is to say,

current science docs not hold that every correlation must be explained via

a common cause. Note, it is clearly not just empirical correlations that

arc to be explained by the principle -- for in the electron spin eases, for

example, we can explain the empirical correlation of the particle detector

states, and Quantum Theory docs so In terms of the states of

unobservables whieh are subject to an Isomorphic correlation. According:

to van Fraassen this limitation on the principle destroys any presumption

that it Is evidential and reduces It to a mere "tacUcal maxim".

Now, I think It Is possible to imagine a theory which could apply this

principle without limit and explain everything. This could be done in a

theory which gave micro-expianations of any correlation, where these

explanations were produced according to some recursively defined

procedure. Theorists could then sequentially explain features of the world

by appeal to ever smaller entities or ever more hidden explanatory factors.

Everything could be explained.

The demand for the complete applicability of the principle of

explanatory power can be represented on the diagram. If one axis of

comparison Is explanatory power then the entire sub-plane is to be

marked out of bounds up to the unit of full explanatory power. This is, thus, an extremely stringent demand on theories.

The alternative Is to leave some things unexplained -- to leave some of

the so-called "cosmic coincidences" in one's theory. Van Fraassen claims

that this is to admit the tenability of the anti-realist position. But surely

there are grades of cosmic coincidences. A theory which posits only a few

sorts of coincidences -- a few sorts of brute facts is certainly better than a

theory which leaves us with a vast number of brute facts. (IGspecially if

the residual brute facts are about what the theory itself proelaims to be

very simple sorts of things, like, needless to say, electrons.) Is it more

likely to be true? It might well seem so, because it is initially likely that,

observable phenomena are in some way the result of hidden activity. And also because it is initially plausible that correlaLions can be explained. If there are some truly brute fact,s t,hls Is perhaps unfortunate from the point, of view of a curious species, bllt It does not seem Impossible. Stili, one

should be forced to accept any fact as brute. The van Fraassen empirical equivalent of a theory T leaves us with so many brute facts which we are not forced to accept, since we have the theory T itself to explain most of them, that this empirieial equivalent can be rejected relative to T.

Van Fraa.'isen has a strikingly simple reply to this sort of argument.

328 William Seager

One ought to believe the more probable of two theories, if one is to bclievc

any at all. The van Fraassen empirical equivalent T' of a theory T is

related to T In the following way: T = 1" + E, where E Is the set of

statemcnts which assert the existence of the various unobservables

mentioned by T. Obviously, then, T' cannot have a lower probability than

T. Indeed, so long a.'l E is not certain, 1" will inevitably be more probable

than T. The way I am looking at this problem, there appears to be something

wrong with this argument. The van Fraassen empirical equivalent T' is

supposed to entail that there arc many sorts of brute facts, but if 'I' = T'

+ E then T must entail that there Is precisely the same set of brute facts. This does not seem right. Brute facts are facts which are not explained by

a theory. For example, the fact that pressure Increases with temperature

(given fixed volume) is a brute fact on the van Fraa.'isen empirical

equivalent of the kinetic thcory of gases. It is not a brutc fact on thc

kinetic thcory itself. So hcrc T docs not seem to bc Just T' + K The tension here might be cxplained in terms of somc kind of non­

standard Implication relation (the counter-factual Implication has the

feature that T's Implying B does not entail that T + X Implies B for

example), or, better, by noting the Intensional character of brute facts

which sterns from the Intensional context "c explains ... ". But another

source of tension Is the question of the rivalry between T and '1". If they

are construed as being incompatible then one cannot be a logical

strengthening of the other (at least if both are consistent). And there is a

natural tendency toward such a view. The anti-realist could be claiming

that the world only has observable features and that his theory is thc

whole story about these. (In fact, van Fraassen admits that the antl­

realist must acccpt many facts as brute which are explicable on a realist

theory Y) Then 'I' is considered to be false by the anti-realist. It is this

anti-realist position we can reject by the above argumcnt. A wcaker anti­rcalist position is a form of agnosticism. It says merely that we will never know that any theory which mentions hidden structure is true. But equally we shall never know that any theory is empirically adequate. The

agnostic anti-realist nevertheless will countenance the acceptance of a theory as empirically adequate In the right evldentlai circumstances. The

difference In attitude must lie in a difference In the support the evidence gives for the two sorts of theory. The van Fraassen empirical equivalent of 'I' is supported by any evidence that supports 'I' itself, and vice versa.

Leibniz and Scientific Realism 329

Where is the difference? The difference Is that the van Fraassen empirical

equivalent of T Is a priori more probable than, or at least as probable as,

T. That this docs not settle the question is obvious from the consideration

that nonetheless the probability of T may be very high. Since within an

agnostic anti-realism the van Fraa.'lsen empirical equivalent of T is not

taken to be a rival of T, an increase in its probability ha.'l no downward

effect on that of T.

To finally draw Lcibniz back into the discussion, consider a principle

he puts forth: -it must be admitted that a hypothesis becomes the more

probable as it is simpler to understand and wider in force and power, that

is, the greater the number of phenomena that can be explained by it.... [t

may even turn out that a certain hypothesis can be accepted as physically

certain if, namely, It complete[y satisfies all the phenomena which occur,

a.'l docs the key to a cryptograph - (L p. 188). The image of th('

crypotgraphy is r('ally perfect here, for as Leibniz admits, it is at least

often possible to find another key which makes sense of the encrypted

message (sec L p. 28:~).

The probability of a theory is then dependent on the number of

incompatible and slgnincantly different empirical equivalents to it. No

one knows whether every theory ha.'l such. [n the absence of competitors

the probability that a theory is true can reach levels where any reasonable

person ought to believe it to be true.

But as observed at the very beginning no extant theory appears to

have reached this degree of probability, despite the glaring lack of

empirically equivalent rivals which an anti-realist might invoke to draw

down a given theory's likelihood. So let me close by considering the

status of science in the real situation of no unqualified empirical success

and a presumption of theory replacement uniformly borne out by the

history of science. Throughout this history th(' gauge of success has be('n

empirical adequacy. But equally pervasive has been the presence of

theories which postulate hidden structures in nature. Each of ttwse

theories is false, but they all imply a truth: there is hidden structure. in this they clearly do better than the van Fraa.'lsen equivaients which merely

assert that each theory is empirically adequate. Nature is subtle. The

postulation is exactly what we would exeet of someone trying to fathom

this subtlety; it is not what we would expect of those seeking only

empirical adequacy. It is because nature has this hidden subtlety that

only theories which posit hidden structure have any hope of empirical

adequacy. [t is reasonable to suppose, then, that scientists are attempting

330 William Seager

to grapple with these hidden features and force them Into the open. This argum~nt is from Lelbnlz, and Is expressed In a beautiful passage:

Unless principles are advanced from geometry and mechanics

which can be applied with equal ease to sensible and insensible things alike, nature In Its subtlety will escape us. And reason

must supply this most important lack in experiment. For a

corpuscle hundreds of thousands of times smaller than any bit

of dust which flies through the air, together with other

corpuscles of the same subtlety, can be dealt with by reason as

easily as can a ball by the hand of a player (L p. 283).

Further, we note that the theories which reason presents to us dealing

with various aspects of nature are all under a constraint of compatibility.

Science rests uneasy whenever powerful theories seem not to go together.

There is no need for this une&'le if empirical adequacy is the object of the

exercise. But if, for example, genetics cannot be integrated Into physics in

some way or other something h&'l gone wrong, even if both are empirically

successful within their own domains. The reply might be made that only

by seeking ever more pncomp&'ising theories will true empirical adeqll~u'y

be achieved. It is true that science seems to seek "total theories", theories

in which every question about nature has an answer. But this cannot be

just the quest for empirical adequacy for this quest ca,nnot result in a total

theory. Such a theory would assert "there is no hidden structure". It is

then a true rival to those theories which do posit hidden structure, and, as

we saw above, Is extremely implausible and can be rejected. I conclude

that the urge to unification in science is a sign that it is the truth which is

being sought and not Just elTlpirieal adequacy.

Department of Philosophy

University of Toronto

Leibniz and Scientific Realism

Notes

1. In L. Loemker (cd.) Philosophical Papers and Letters, Reidel, 1956, p. 363. Henceforth I will refer to the Loemker volume as "1.,".

2. Van Fraassen, B. The Scientific Image. Clarendon Press, Oxford, 1980, p. 12.

3. In M. Hooker (ed.) Leibniz: Critical and Interpretive Essays. University of Minnesota Press, 1982, pp. 3-20.

4. G.II.R. Parkinson, ed. Philosophical Writings, p. 85. 5. For more on the general operation of the metaphysical

principles see my "The Principles of Continuity and the

Evaluation of Theories", Dialogue, vol. 20, 3, 1981. 6. See R. Boyd, "Realism, Underdetermination and a Causal

Theory of Evidence" In Nous, March 1973 and "SclenW1c

Realism and Naturalistic Epistemology" in Proceedings of the 1980 Biennial Meeting of the Philosophy of Science Association, Volume Two (cd. P. Asquit.h and R. Giere), Philosophy of Science Association, 1981.

7. The Scientific Image. p. 31.

8. Ibid, p. 23.

9. Ibid, p. 24.

331