Scientific hypotheses are refuted, or at least called seriously into question, when predictions based on them fail to come true. As Karl Popper—who has emphasized that there is a unity of method in all generalizing or theoretical sciences—has insisted, every scientific hypothesis must be testable, and the way to test it is to look for circumstances in which it does not hold. To content oneself with favourable evidence is not enough; one must be searching all the time for unfavourable evidence. Further, it must be possible, if the hypothesis is genuinely scientific, to specify in advance what would count as unfavourable evidence; the circumstances in which the hypothesis needs to be abandoned, or at least modified, must be indicated precisely. In ideal conditions it is possible to devise a crucial experiment that will test a hypothesis definitively; the Michelson–Morely experiment, which disposed of the theory of the luminiferous ether, was such an experiment.
It can be asked, however, what parallels there are to this in metaphysics. The difficulty with testing a metaphysical thesis is twofold. First, metaphysical theories tend to be extremely general and as such highly unspecific. They announce, for example, that every event has some cause or other, or that every change is part of a process that serves some purpose. To find counterexamples to theses of such generality is on any account exceedingly difficult: how can one be sure that all the possibilities have been explored? There is, however, another and still more serious difficulty. The scientist, once he has laid down the conditions that would have to obtain for his hypothesis to prove false, makes no bones about their occurrence; it is, typically, a matter of whether or not a certain pointer reading is registered, and this is a simple question of ascertainable fact. Fact for the metaphysician, however, is altogether more slippery. Different metaphysicians see the world each in his separate way; what they take to be the case is coloured by their metaphysical conceptions. There is no neutral body of facts to which appeal can be made to show that a metaphysical theory falls down, and this being so, the attempt to assimilate metaphysics to science must fail.
That this should be the case is perhaps not surprising. Scientific thinking proceeds within a framework of presuppositions that it is the business of the scientist to use, not to argue for and still less to challenge—presuppositions to the effect, for example, that every change has a natural explanation. No doubt scientists can change their presuppositions, but they seldom do so consciously; their usual practice is to take them for granted. Metaphysicians, however, necessarily take a very different attitude toward presuppositions. It is their business to tell men how to understand the world, and this means that they must, among other things, put forward and argue for a set of interpretative principles. Metaphysicians differ radically in the interpretative principles they accept, and it is this that explains their failure to agree upon what to take as fact. It is naïve to suppose that the points at issue between, for example, a Thomist and a Materialist can be settled by observation or even by experiment; the facts to which one might appeal in support of his theory may be seen in a very different light by the other, or perhaps be dismissed as simple illusion. Reflection on the phenomenon of religious experience will illustrate what is meant here. That men undergoing this experience are affected mentally and physically in certain specific ways is perhaps common to both Thomist and Materialist. But the further description of their state is entirely controversial and owes its controversial character to the varying preconceptions that the disputants bring to their task. Initial metaphysical insights Origin
If metaphysics is far from being a simple empirical discipline, however, it does not follow that it is wholly without foundation in fact. The true situation can perhaps be put as follows. Every metaphysic consists in an imaginative view of the world elaborated into a conceptual system. Metaphysics, like poetry, begins by being a matter of vision; a metaphysician sees the scheme of all things in a certain light; for example, as nothing more than a vast mechanism or as God’s creation. As a metaphysician, however, he cannot be content to rest in a vision of this sort, as for example the Romantic poet William Wordsworth does in his “Intimations of Immortality.” He needs to think out terms in which whatever exists can be described so as to accord with his primary insight; he needs to produce and apply a conceptual system and to argue against possible alternatives. Whatever its origins, metaphysics is strictly intellectual in its development. When the question is raised of the source from which metaphysicians gain their initial insights, the answer that occurs most readily is that they are derived from reflection on certain evident facts. Thus, the source of the Materialist view of the world is undoubtedly the practice of science; the Materialist proposes to give unrestricted validity to ways of thinking that scientists have found effective in a certain restricted sphere. The source of Idealist thought is to be found in the practice of history, or more generally in the interpersonal relations of beings who are at once rational and sensitive; the Idealist philosopher takes concepts that are appropriate in these limited areas to apply to the whole of reality. Every system of metaphysics is grounded in some real experience and owes its initial appeal to that fact. This is not to say, however, that the metaphysician builds on experience as does his scientific colleague. To think that is to take altogether too simple a view of the whole question. Tests of validity
A question of immense importance is whether there are any means of comparing the validity of initial metaphysical insights. If it has to be answered negatively—if it has to be allowed that, as it were, all candidates in this field start and finish on an equal footing—the argument that each of them has a foundation in fact will be entirely discounted. Whatever respectability their concepts possess in their original homes will be lost once they fall into the hands of the metaphysician, because the procedure of the latter in taking them up and extending them is essentially arbitrary. For example, that one sees the sum of things as a vast machine may be suggested by what goes on in science, but this view can neither claim scientific warrant itself nor draw on scientific prestige, because it seems to spring from nothing better than mere whim. There are, however, two reasons for thinking that initial metaphysical insights are based not on mere whim but on valid grounds.
First, the number of what may be called viable metaphysical insights is in practice limited: there are varying ways of taking the world as a whole, but not an infinite variety. In the outline account of metaphysical theories given above, six different kinds of view were distinguished, each of which may be said to be grounded in one or more areas of experience. It would be possible to extend the list, but probably not very far; further candidates might well turn out to be no more than variations on themes already considered. Thus, Leibniz might be seen as a latter-day Platonist, and Spinoza as offering a different version of the dualism of Descartes, one that is more sympathetic to Materialism than was Descartes himself. If these claims are true, they are certainly important; for the facts here adduced suggest that the experiences or visions on which different metaphysicians build are not peculiar to individual minds but occur commonly and regularly. They are not the product of passing moods, seized on and exploited for no good reason, but connect with thoughts that recur repeatedly in sensitive and intelligent reflection.