Aside from these main positions, an interesting development is the stress laid by writers—such as Stuart Hampshire, an “ordinary language” philosopher—on self-activity as the distinguishing characteristic of mind. According to this view, a human being is a body among bodies but is, as Plato said, self-moving as material things are not. That this should be so—that human beings are possessed of wills and can in favourable circumstances act freely—is taken as an ultimate fact neither requiring nor capable of explanation. It is often denied that any scientific discovery could give rational grounds for questioning this fact. It is also stressed that the causality of a human being is fundamentally different from that of a natural subject, intentional action being quite other than mere behaviour determined from without.
Connected with these topics is the problem, much discussed in recent philosophy as a result of the rise of cybernetics, of what differentiates men from machines. Two answers used to be given: the power to think and consciousness. Now, however, there exist machines whose calculating abilities far surpass those of any human being; such machines may not literally think, but they certainly arrive at conclusions. Furthermore, it is not true that their operations are of a purely routine nature: there is a sense in which they can improve their performance in the light of their “experiences.” They even have an analogue of consciousness in the sensitivity they show to external stimuli. These facts suggest that the gap between minds and machines is less wide than it has often been thought to be; they do not, however, destroy it altogether. Human beings possess powers of creative thought unlike anything found in machines; as Noam Chomsky, an American linguistics scholar, has stressed (and as Descartes urged in his Discours de la méthode), the ability of human beings to handle language in such a way that they comprehend any one of an infinite number of possible expressions is something that cannot be explained in mechanical terms. Again, as J.R. Lucas, a British philosopher, has argued, human beings have the ability to diagnose and correct their own limitations in a way to which there is no parallel in machines. As some older philosophers put it, man is a being with the power of self-transcendence; he can work within a system, but he can also move to another level and so see the shortcomings of the system. A machine can only work within a system; it operates according to rules but cannot change them of its own accord.
Finally, mention should be made of an extreme Materialist solution to the mind–body problem: this solution holds that states of mind are in fact states of the brain. Supporters of this theory agree that the two are separate in idea but argue that physiology shows that despite this they are contingently identical. What seems to be a state of mind, above all to its possessor, is really a state of the brain, and mind is thus reduced to matter after all. It is not clear, however, why physiologists should be granted the last word on a topic like this, and, even if it were agreed that they should be, the correlations so far established between mental occurrences and states of the brain are at best sketchy and incomplete. Central-state Materialism, as this theory is called, professes to have the weight of contemporary science behind it, but it turns out in fact to have drawn to a remarkable degree on what it thinks will be the science of tomorrow. Nature and the external world
The problem of the existence of material things, first propounded by Descartes and repeatedly discussed by subsequent philosophers, particularly those working within the Empiricist tradition, belongs to epistemology, or the science of knowledge, rather than metaphysics; it concerns the question of how it can be known whether there is a reality independent of mind. There are, however, problems about nature and the external world that are genuinely metaphysical. The reality of material things
There is first of all the question of the status, or standing, of material things, the kind of being they possess. It has been repeatedly suggested by metaphysical philosophers that the external world is in some way defective in reality, that it is a mere phenomenon, something that seems to be what it is not. Plato, as has already been pointed out, held that objects of the senses generally answered this description; they each appeared to possess characteristics that they could not in fact have (water could not be at once hot and cold) and were to that extent delusive rather than real. There was no stability in the world of phenomena and therefore no true reality. In taking this view, Plato drew no contrast between the world of nature and the world of man, although he undoubtedly believed that souls had a superior status. Leibniz, a later philosopher who also followed this general line of thought, began by explicitly opposing souls to material things. To speak precisely, nothing truly existed except monads, and monads were souls, or spiritual beings: all had perceptions, although these varied enormously in degree of clarity (the perceptions of the monads constituting what is commonly called a stone were singularly faint). Although the final description of the world must thus be given in mental terms, it did not follow that nature as normally perceived is a total illusion. Men perceive as well as think, and, although perception is in fact simply a confused form of thought, it is not for that reason to be set aside altogether. The world of nature, the world of things in space and time, is, as Leibniz put it, a “well-founded phenomenon”; it is what all men must judge to be there, given that they are not pure intellects but necessarily remain to some extent prisoners of their senses.
A theory on somewhat similar lines was worked out by Kant in the Kritik der reinen Vernunft (1781; Critique of Pure Reason), despite Kant’s explicit dissent from Leibniz’ account of perception as confused thinking. Kant contrasted a realm of things as they are in themselves, or noumena, with a realm of appearances, or phenomena. The former are unknown, and indeed unknowable, though it seems clear that Kant tended to think of them on lines like those of Leibniz; phenomena do not exist independently but are dependent on consciousness, though not on any one person’s consciousness. Kant expressed this position by saying that things phenomenal are empirically real but transcendentally ideal; he meant that they are undoubtedly there for the individual subject, though when examined from the point of view of critical philosophy, they turn out to be conditioned by the mind through the forms of sensibility and understanding imposed upon them. Kant’s most striking argument for this conclusion was that space and time are neither, as the English physicist Sir Isaac Newton supposed, vast containers inside which everything empirical is situated nor, as Leibniz had suggested, relations between things confusedly apprehended but are rather what he mysteriously called “pure intuitions,” factors inherent in the sensibilities of observers. Without observers space and time disappear along with their contents; but once the human point of view is assumed, in the form of percipients who are directly aware of the world through their senses, space and time become as real as anything—indeed, more real because of their pervasive character. There is nothing that falls within experience that does not have temporal relations, and all the data of the senses have spatial relations as well.