The simplest form of Materialism is found in the claim that only matter exists. Stated thus baldly the claim is absurd, because it is clear that all sorts of things exist that are not of the nature of matter: thoughts and numbers and human institutions would be instances. In the light of these facts, the claim has to be revised to say that matter is the only substantial existent, with appeal being made to distinctions first worked out in Aristotle’s doctrine of categories. According to this explanation, many things besides matter exist, but all of them are explicable (or so it is said) as modifications of matter. Thus, human institutions consist in patterns of movement among specific groups of human beings, and human beings in turn are nothing but highly complicated material bodies.
It is clear from these instances that Materialism is a controversial doctrine; it is also clear that its key word, modification, requires further explanation. When, for example, minds are said to be modifications of an underlying material substance, what is meant? A first and relatively easy point is that, like qualities and quantities, they could not exist separately. Unless there were material bodies, there could not be minds, because minds are—to put it crudely—states found in some material bodies. Minds are here equated with mentality, and mentality is clearly an abstraction. To say this, however, is not to remove the whole difficulty. When it is said that mentality is a state of some material body or bodies, is that meant literally or metaphorically? Bodies can often be described from the physical point of view as being in a certain state—for example, as being in a state of internal equilibrium. What is meant here is that the different particles of matter concerned stand in a certain relationship and as a consequence develop certain physical properties. But is mentality to be conceived as a physical property? It sounds extravagant to say so. Yet some such doctrine must be defended if Materialism is to be advanced as a form of ontology with a serious claim for attention. It is interesting in this connection to notice the arguments advanced by scholars like J.J.C. Smart, which purport to identify states of mind with states of the brain. If the two are identical—literally the same thing described from two points of view—thoughts may really be modifications of matter, and Materialism may be tenable in a strong form. If, however, the identity cannot be made out—and very few philosophers are in fact ready to accept it—Materialism can be true at most in a modified form.
This modified form of Materialism is perhaps better described as naturalism. Naturalism holds not that all things consist of matter or its modifications but that whatever exists can be satisfactorily explained in natural terms. To explain something in natural terms is to explain it on scientific lines; naturalism is in fact a proclamation of the omnicompetence, or final competence, of science. It is not essential to this type of view to argue that phenomena can be spoken of in one way only; on this point, as on the point about ontological reducibility, the theory can afford to be liberal. It is, however, vital to make out that the scientific account of a set of happenings takes precedence over any other. Thus, the language in which men commonly speak of action and decision, which may be called for short the language of reasons, must be held to be secondary to the language in which scientists might speak of the same facts. Scientific language is basically causal, and the thesis of this form of Materialism is that causal explanations are fundamental. Naturalism is thus the obverse of Hegelianism; it is a theory of first principles, and it draws its principles from science.
If the question is raised why anyone should take this form of Materialism seriously, the answer lies in a number of significant facts. Physiologists have established correlations between general states of mind and general states of brain activity; their hope is to extend this to the point where particular thoughts and feelings can be shown to have their physiological counterpart. Cyberneticists have produced artifacts that exhibit mindlike behaviour to a remarkable degree; the inference that man is no more than a complicated machine is certainly strengthened by their achievements. Sociologists have shown that, whatever the explicit reasons men give for their beliefs, these are often intelligible in the light of factors of which they themselves take little or no account. The old assumption that human judgments are typically grounded in reason rather than merely caused, is called in question by the results of such investigations, which gain support from findings both in Freudian and in orthodox psychology. None of this evidence is decisive by itself; there are ways in every case of blocking the conclusions that Materialists tend to draw from it. Yet it remains true that, cumulatively, the evidence is impressive. It certainly has enough force to make it necessary to take this type of theory with the greatest seriousness. Metaphysical disputes in the modern world are fundamentally arguments for or against Materialism, and the other types of theory here explored are all seen as alternatives to this compelling, if often unwelcome, view. Argument, assertion, and method in metaphysics
Attention is now turned from description of the content of particular metaphysical views to more general treatment of the nature of metaphysical claims. The questions that will arise in this section concern such things as the nature and basis of metaphysical assertions, the character of metaphysical arguments and of what are taken to be metaphysical proofs, and the parts played in metaphysical thinking by insight and argument, respectively. They come together in the inquiry as to whether metaphysics can be said to be a science and, if so, what sort of a science it is. Metaphysics as a science Nature of an a priori science
Sciences are broadly of two kinds, a priori and empirical. In an a priori science such as geometry, a start is made from propositions that are generally taken to be true, and the procedure is to demonstrate with rigorous logic what follows if they are indeed true. It is not necessary that the primary premises of an a priori science should in fact be truths; for the purposes of the system they need only be taken as true, or postulated as such. The main interest is not so much in the premises as in their consequences, which the investigator has to set out in due order. The primary premises must, of course, be consistent one with another, and they may be chosen, as in fact happened with Euclidean geometry, because they are thought to have evident application in the real world. This second condition, however, need not be fulfilled; a science of this kind can be and commonly is entirely hypothetical. Its force consists in the demonstration that commitment to the premises necessitates commitment to the conclusions: the first cannot be true if the second are false.