In his

*Two New
Sciences* (1638), Galileo presents a puzzle about infinite collections of
numbers that became known as ‘

Galileo’s paradox’. Written in the form of a
dialogue, the interlocutors in the text observe that there are many more positive
integers than there are perfect squares, but that every positive integer is the
root of a given square. And so, there is a one-to-one correspondence between the
positive integers and the perfect squares, and thus we may conclude that there
are as many positive integers as there are perfect squares. And yet, the
initial assumption was that there are more positive integers than perfect
squares, as every perfect square is a positive integer but not vice-versa; in
other words, the collection of the perfect squares is strictly contained in the
collection of the positive integers. How can they be of the same size then?

Galileo’s conclusion is that principles and concepts
pertaining to the size of *finite*
collections cannot be simply transposed, mutatis mutandis, to cases of
infinity: “the attributes "equal," "greater," and
"less," are not applicable to infinite, but only to finite,
quantities.” With respect to finite collections, two uncontroversial principles
hold:

**Part-whole**: a
collection A that is strictly contained in a collection B has a strictly
smaller size than B.

**One-to-one**: two
collections for which there exists a one-to-one correspondence between their
elements are of the same size.

What Galileo’s paradox shows is that, when moving to
infinite cases, these two principles clash with each other, and thus that at least one
of them has to go. In other words, we simply cannot transpose these two basic
intuitions pertaining to counting finite collections to the case of infinite
collections. As is well known, Cantor chose to keep **One-to-one** at the expenses of **Part-whole**,
famously concluding that all countable infinite collections are of the same
size (in his terms, have the same cardinality); this is still the reigning
orthodoxy.

In recent years, an alternative approach to measuring infinite sets is being developed by the mathematicians Vieri Benci (who initiated the project) Mauro Di Nasso, and Marco Forti. It is also being further explored by a number of people – including
logicians/philosophers such as Paolo Mancosu, Leon Horsten and my colleague
Sylvia Wenmackers. This framework is known as the theory of numerosities, and
has a number of theoretical as well as more practical interesting features. The
basic idea is to prioritize

**Part-whole**
over

**One-to-one**; this is accomplished
in the following way (

Mancosu 2009, p. 631):

Informally the
approach consists in finding a measure of size for countable sets (including
thus all subsets of the natural numbers) that satisfies [**Part-whole**]. The new ‘numbers’ will be called ‘numerosities’ and
will satisfy some intuitive principles such as the following: the numerosity of
the union of two disjoint sets is equal to the sum of the numerosities.

Basically, what the theory of numerosities does is to
introduce different *units*, so that on these new units infinite sets comes out
as finite. (In other words, it is a clever way to turn infinite sets into
finite sets. Sounds suspicious? Hum…) In practice, the result is a very robust,
sophisticated mathematical theory, which turns the idea of measuring infinite
sets upside down.

The philosophical implications of the theory of numerosities
for the philosophy of mathematics are far-reaching, and some of them have been
discussed in detail in (

Mancosu 2009). Philosophically, the mere fact that there
is a coherent, theoretically robust alternative to Cantorian orthodoxy raises
all kinds of questions pertaining to our ability to ascertain what numbers
‘really’ are (that is, if there are such things indeed). It is not surprising
that Gödel, an avowed Platonist, considered the Cantorian notion of infinite
number to be inevitable: there can be only one correct account of what infinite
numbers

*really* are. As Mancosu points
out, now that there is a rigorously formulated mathematical theory that
forsakes

**One-to-one **in favor of

**Part-whole**, it is far from obvious that
the Cantorian road is the inevitable one.

As mathematical theories, Cantor’s theory of infinite
numbers and the theory of numerosities may co-exist in peace, just as Euclidean
and non-Euclidean geometries live peacefully together (admittedly, after a
rough start in the 19^{th} century). But philosophically, we may well
see them as competitors, only one of which can be the ‘right’ theory about
infinite numbers. But what could possibly count as evidence to adjudicate the
dispute?

One motivation to abandon Cantorian orthodoxy might be that
it fails to provide a satisfactory framework to discuss certain issues. For
example,

Wenmackers and Horsten (2013) adopt the alternative approach to treat certain
foundational issues that arise with respect to probability distributions in
infinite domains. It is quite possible that other questions and areas where the
concept of infinity figures prominently can receive a more suitable treatment
with the theory of numerosities, in the sense that oddities that arise by
adopting Cantorian orthodoxy can be dissipated.

On a purely conceptual, foundational level, the dispute
might be viewed as one between **Part-whole**
and **One-to-one,** as to which of the
two is the most fundamental principle when it comes to counting *finite* collections – which would then be
generalized to the infinite cases. They are both eminently plausible, and this
is why Cantor’s solution, while now widely accepted, remains somewhat
counterintuitive (as anyone having taught this material to students surely
knows). Thus, it is hard to see what could possibly count as evidence against
one or the other

Now, after having thought a bit about this material
(prompted by

two wonderful talks by Wenmackers and Mancosu in Groningen
yesterday), and somewhat to my surprise, I find myself having a lot of sympathy for
Galileo’s original response. Maybe what holds for counting finite collections
simply does not hold for measuring infinite collections. And if this is the
case, our intuitions concerning the finite cases, and in particular the
plausibility of both

**Part-whole** and

**One-to-one**, simply have no bearing on
what a theory of counting infinite collections should be like. There may well
be other reasons to prefer the numerosities approach over Cantor’s approach (or vice-versa),
but I submit that turning to the idea of counting finite collections is not
going to provide relevant material for the dispute in the infinite cases. In fact, from this point
of view, an entirely different way of measuring infinite collections, where
neither

**Part-whole** nor

**One-to-one** holds, is at least in
principle conceivable. In what way the term ‘counting’ would then still apply
might be a matter of contention, but perhaps counting infinities is a totally
different ball game after all.