“When one man speaks to another man who doesn't understand him,and when the man who's speaking no longer understands, it's metaphysics”. -Voltaire, Candide, 1759
Falsifiability is the criterion formulated by Karl Popper to delimit the range of controllable theories, that pertains to science, from that of the theories that can not be controlled, as Popper himself identified metaphysics.
Then, I believe it is worthwhile asking: is metaphysics a science?
I’ll begin coming at the question quoting the words of Craig Callender who said that “many debates in analytic metaphysics are sterile or even empty while also believing that metaphysics is deeply infused within and important to science”. (C. Callender, “Philosophy of Science and Metaphysics”, To be submitted to the Continuum Companion to the Philosophy of Science).
Metaphysics, for Popper is no longer a set of theories and beliefs meaningless, as the neo-positivism or general study of ultimate and transcendent truths. It is simply any theory (with meaning and significance) that is not science because it is never falsifiable
(but which can, on occasion, come to the aid of science and research, by providing ideas and perspectives to frame the issues). Popper used in his lectures the expression ' metaphysical research programs’, topic of a Postscript that Lakatos transformed into scientific research programs.
Indeed, Popper showed that the "verification" is not enough when you want to ensure the truth of a scientific theory. In fact, the verified theories succeeded with neither of them could acquire infallibility once and for all. And, reversing the apparent evidence according to which the scientific theory would be the bearer of certainty, Popper showed that, instead, the proper characteristic of a scientific theory consists in "fallibilism".
As it is known, the Popperian concept of falsifiability is opposed sharply to that of neo-positivist verifiability, aimed at defining a criterion of sense (significant, that is say something, only assertions inductively verifiable; assertions of metaphysics, which are not, are not significant).
The criterion of falsifiability says, then, that a theory, to be controllable, and therefore scientific, must be "falsifiable": in logical terms, from its basic premises it must be able to be deductible the conditions of at least one experiment that could prove the theory entirely false, according to the logical process of the modus tollens (according to which, if we deduce B from A, and if B is false, then A is also false). If a theory does not own this property, it is impossible to check the validity of its information content regarding the fact that it presumes to describe.
According to the classical interpretation of Popper, psychoanalysis as well as dialectical materialism, historicism (and I would add metaphysics) which presented themselves as scientific theories, are – in fact - pseudoscience because do not meet the basic requirement of experimental controllability (verificationism) . That is at the base of the scientific method. These are known as "highly explanatory theories" in the sense that they seem to have a valid explanation for each phenomenon taken into consideration.
However, this is also their weak point, as their explanatory power discharges any empirical test reproducible experimentally; that is, to use the terminology of Popper, are not "falsifiable". According to the Austrian epistemologist, in fact, every scientific theory must provide the criteria by which it can be controlled, questioned and possibly refuted on an experimental basis (falsification).
The Popper’s reflection on metaphysical issue confused the neo-positivists for whom metaphysical theories were nothing more than 'the fruits of not- reason’. Based on the principle of verification that have only sense propositions empirically verifiable, the attitude of the Vienna Circle was that of a rejection of metaphysics. Wittgenstein was hard in saying that: most of the propositions and questions that are written in the field of philosophy, are not false, but senseless (prop 4,003) . Wittgenstein's attack to philosophy has been configured as a tough and 'authoritative' assault on metaphysics. (D. Antiseri, “Ragioni della razionalità, proposte teoretiche”, Rubettino, Vol. 1, 2004, 96)
Each of metaphysical theories worked very first to become controllable; each of them operated as a program of science. From the historical perspective there have been metaphysical theories from which scientific theories flowed (Antiseri, 147). Metaphysics can play an important role in generating scientific hypotheses. It has even been said that "What begins as metaphysics, can end up as science”(P.F. Strawson, 1960, 110). Metaphisics offer new perspectives; it is neither true nor false (Antiseri, 109). Metaphysics is outside and beyond the empirical verification (otherwise it would be an empirical science).
Then, based on the Peirce thought, I would consider philosophy (at her place I’d put metaphisics) scientific not because it is a science in itself, but because it is close and in harmony with science, and simultaneously capable of illuminating its meaning (Antiseri, 125)
Thus, as a way of a summary, I’d conclude by saying that metaphysics, not wanting to be a set of tautologies, is necessarily a pseudo- scientific theory. A metaphysical theory can be seen as a research program. The difference between a metaphysical theory and a pseudo-scientific one is that the former is fruitful of predictive theories that foresee facts and exclude others, while a pseudo-science gives birth only (or almost only) to ‘ad hoc’ hypotheses that can not exclude any fact. The pseudo science grows ‘ as a cancer’ on itself. The dividing line between metaphysics and pseudoscience is their fertility or infertility against refutable theories they generate.
Epistemological reflection on the relationship between metaphysics and science has created an arsenal of conceptual tools (falsifiable theories and theories which are not, metascientific paradigms, research programs for decision rendered unforgeable by a methodological decision, distinction between metaphysics and pseudo-science, and so on. (Antiseri, 187-188).
So, I’d say with Callender that there “are plenty of significant areas of metaphysics in which to work, philosophers are needed for this work, and one hopes that they can sometimes make a distinctive positive contribution, philosophers but not physicists are currently doing some of the necessary systemization of the physical world”.
Then, it is not falsification in itself that is necessary for the advancement of science, but the spirit of research, cooperation and mostly human ingenuity.