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The issue over empiricism and realism, presently the focus of much discussion in the philosophy of science, is but the manifestation of an age-old perplexity. The perplexity is over the relation between one’s experiences, and the world, or between phenomena and reality.

The debate over empiricism and realism concerns the very nature of modern science: what it is, or what it ought to be. Empiricism, in its extreme form, claims that there is no reality behind appearances, and that it is the task of science to determine what the appearances are and what the formal relations are that obtain among them. In its weaker form, in which the existence of a trans-empirical reality is admitted, empiricism claims that due to the transcendent nature of this reality no knowledge can be had of it, and it suggests essentially the same methodology as does the stronger form.

Through the years empiricists and realists have been battling over the question of whether or not science should be restricted to investigating what is observable, with neither party paying virtually any attention to the question whether the observable is to be conceived differently from their respective viewpoints. Phenomenalists have argued that phenomena alone should be treated as actually existing, assuming that the empirical laws of science in fact link phenomena in their sense of the term. And realists, for their part, have advocated that science recognise a trans-empirical realm, without considering whether the empirical realm takes a form more in keeping with their view or that of the empiricists or phenomenalists.

Though measurement and the discovery of empirical laws presuppose the adoption of an epistemological realism, they leave untouched that aspect of realitywhich empiricists have been most concerned to avoid, namely causes that are actually productive of their effects. In the belief that such causes are truly unknowable, empiricists have either forbidden their investigation, or replaced them with ‘causes’ that are simply manifestations of the principle of the uniformity of nature, i.e. constant conjunctions. The aspect of modern science we shall consider now is precisely that which investigates the nature of real causes, and it is thus fundamentally realist in its orientation. In its paradigmatic form it involves the construction of theories intended to explain empirical laws by indicating both the regular causes underlying them, as well as how those causes operate in such a way as to be contiguous with their effects.

Modern science has two basic tasks: one, the determination of what the facts are, and the other, the explanation of why they are as they are. The determination of the facts is paradigmatically accomplished through the establishment of empirical laws, the expressions ofwhich are seen to provide scientific knowledge, while the explanation of the facts is performed by theories, which provide an understanding of them. The determination of what the facts are presupposes the principle of the uniformity of nature, and their explanation consists in showing how they are but manifestations of the principles of the discipline taken as a whole, and in particular of the contiguity principle of causality. The parameters within which either knowledge or understanding can be said to have been attained are set by the principle of substance.

In Chapters 2 to 4 above the attempt was made to characterise the essential features of modern science; and in Chapter 5 the PTL model of scientific explanation was presented against the background of this characterisation. The three key notions in this approach to science are principles, laws and theories; and the principles concerned have been specified as those of uniformity, substance and causality. These principles, conceived of in their paradigmatic modern-scientific form, imply strict determinism, a perpetually existing physical substance, and the operation solely of efficient causes which are contiguous with their effects.

The world as we experience it is not a constantly changing mosaic of sensations in which no order can be discerned, but one which evinces a particular regularity from day to day and year to year. For modern science this regularity rests on the principle of the uniformity of nature, which states that natural change is lawful or takes place according to rules. In conjunction with the other two principles fundamental to modern science, it claims that changes in the world are the result of causes contiguous with their effects operating in a regular fashion on a perpetually existing substance. In this way modern science attempts to explain change in terms of non-change: perpetually existing (types of) causes operate on a perpetually existing substance in a regular manner.

Various theories of probability have been propounded in modern times. In the present chapter we shall consider the most important of them with regard to their applicability to modern science, and suggest a particular view – in which the nominal and real aspects of reality are distinguished – to be the superior alternative.We shall also consider the application of probabilistic and similar forms of thinking to questions regarding the respective acceptability of theories, laws and principles.

The debate in the philosophy of science between empiricism (positivism) and realism is not so much a debate concerning howscience is or has been practised, as one concerning how it to be practised. Empiricists, for their part, view the aim of science as the affording of truth, and want therefore to exclude from science any activity of a hypothetical nature. Realists, on the other hand, see the aim of science as concerning understanding as well as truth, and view informed speculation about the nature of the real world as a worthwhile attempt to obtain such understanding.

In the debate between realism and empiricism the question of the viability of modern science is seldom raised, the issue being rather one of how modern science ought best be conducted. In fact few of the contributions to the debate, whether realist or empiricist, even recognise that fundamentally different alternative forms of science are possible, the belief being rather that with the Scientific Revolution of the seventeenth century, if not earlier, humankind hit upon the one true path to knowledge, the task now being that of continuing to follow that path in the best possible way.