. The traditional distinction between ‘prescriptive’ laws (legal, moral, divine) and ‘descriptive’ laws (scientific) is convenient, but not necessarily accurate. ‘Prescriptive’ laws (see philosophy of LAW) may not be prescriptions, and ‘descriptive’ laws may not describe the world. The laws of logic and mathematics are simply accepted statements, usually important, in those subjects. The laws of thought are the logical laws of identity, contradiction and EXCLUDED MIDDLE. Whether they are more important than other logical laws is disputed. The rest of this entry mainly concerns scientific laws.
Generalizations may be closed (limited in space and time: ‘All the coins now in my pocket are silver’) or open (‘All ravens, at all times and places, are black’). But on another interpretation open generalizations have the form, ‘Anything whatever, if it is so-and-so, is such-and-such’, where ‘so-and-so’ may or may not contain spatiotemporal restrictions like ‘now in my pocket’. On this interpretation both the first two examples would be closed, because they each have a limited subject-matter (coins and ravens), and any generalization could be formulated as an open generalization of the second kind.
In so far as scientific laws are generalizations, they are usually regarded as open on the first interpretation (though this excludes those of Kepler (see below) and Galileo). They also are usually taken to imply counterfactuals (see CONDITIONALS): ‘All ravens are black’, if a law, implies ‘If there were ravens on Mars (though there aren’t) they would be black’. For both these reasons laws cannot be conclusively verified. Also many laws seem to have no direct application: ‘All bodies unacted on by forces move with constant velocity in a straight line’—but there are no such bodies. For these and other reasons scientific laws are sometimes thought to be rules governing the scientist’s expectations, and so prescriptive, or else idealized descriptions to which the world approximates, as triangles on a blackboard approximate to Euclidean triangles. On this last view the point of Newton’s first law of motion (quoted above) is that any deviation of an object from uniform rectilinear motion must be attributed to its being acted on by forces. Some writers refuse to call laws ‘true’ on the grounds that they are not straightforwardly descriptive.
Normally a hypothesis is a statement not yet accepted as true, or as a law, while a law is only called a law if it is accepted, whether or not we call it ‘true’. But occasionally laws, though accepted, are still called hypotheses, e.g. ‘Avogadro’s hypothesis’. A lawlike statement is sometimes a statement resembling a law except that it is not accepted and is perhaps rejected, and sometimes a statement not general enough to be a law because it refers to individual objects (e.g. Kepler’s ‘laws’ about how ‘the’ planets go round ‘the’ sun, which do not mention suns and planets in general). Occasion-ally it is a statement attributing dispositional characteristics, e.g. ‘Glass is brittle’.
Theory has various meanings: (i) One or more hypotheses or lawlike statements (either of first two senses), regarded as speculative. (ii) A law about unobservables like electrons or evolution, sometimes called a theory because evidence about unobservables is felt to be inevitably inconclusive, (iii) A unified system of laws or hypotheses, with explanatory force (not merely like a railway timetable). (iv) A field of study (e.g. in philosophy: theory of knowledge, logical theory). These senses sometimes shade into each other.
A principle may be a high-grade law, on which a lot depends, or it may be something like a rule. To call all scientific laws principles suggests they hover between being rules and being idealized descriptions. Legal, moral, aesthetic, etc., principles may resemble scientific laws in being descriptions of ideal worlds, set up to govern actions as scientific laws are to govern expectations. However, they differ from them by not being idealized descriptions of the real world, to be rejected unless the real world approximates to them in the relevant ways. (Other uses of ‘law’ and ‘principle’ exist.)
Scientific laws are often called laws of nature or natural laws. Natural law (generic singular) is the moral law (i.e. set of laws) regarded as derivable from the general nature of the universe by reason alone, without appeal to revelation, feelings, interests, etc. See also EXPLANATION, MODALITIES, CONVENTIONALISM, INSTRUMENTALISM.
D.M.Armstrong, What is a Law of Nature?, Cambridge UP, 1983. (A connection between universals.)
R.B.Braithwaite, Scientific Explanation, Harper, 1953. (Pp. 300–3 analyse scientific laws in terms of their explanatory function.)
J.W.Carroll, Laws of Nature, Cambridge UP, 1994. (Defends realist view of them, connecting with causation.)
A.P.D’Entreves, Natural Law, Hutchinson, 1951, 2nd edn republished with new introduction by C.J.Nederman, Transaction Publishers 1994. (Sympathetic discussion, more historical and less analytical than Finnis.)
J.Finnis, Natural Law and Natural Rights, Clarendon, 1980. (Full discussion from mainly philosophical rather than historical point of view. Cf. also, for an anthology, J.Finnis (ed.), Natural Law, 2 vols, Dartmouth Publishing Company, 1991.)
W.Kneale, Probability and Induction, Oxford UP, 1949. (Part 2 discusses various kinds of scientific law, and claims that they express objective necessities.)
M.Singer, Generalization in Ethics, Eyre and Spottiswoode, 1963, chapter 5. (Moral rules and principles).
S.E.Toulmin, Philosophy of Science, Hutchinson, 1953. (Advocates ‘idealized description’ view, though without so calling it, and discusses other views.)
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